TELEVISION : Sunday evenings once belonged to Ed Sullivan, then ‘Maverick’ and ‘All in the Family.’Now the four networks are squaring off at 8 o’clock with costly challengers to the reigning queen; yes, the stakes are enormous : The Sunday Night Fights

In May, ABC Entertainment President Ted Harbert was in New York with his fellow network executives, collectively scratching their heads and trying to figure out how best to lay out their schedule of fall TV programs, when a most unusual telephone call came to his room at the Regency Hotel.

The call, according to sources at ABC, was from an anonymous NBC executive who’d been directed to deliver a message: “The NBC network wants you to know, before you set your fall schedule, that we plan to put ‘seaQuest’ on Sunday at 8 p.m.”

ABC had been eyeing that time slot for a new romantic drama, “Lois & Clark: The New Adventures of Superman,” which, like NBC’s underwater adventure series from executive producer Steven Spielberg, carried a big budget, big special effects and big merchandising hopes--and was aimed at the same family audience.

“I suppose they thought that would scare us out of there,” said a senior ABC executive.

No such luck. ABC went ahead and slotted “Lois & Clark” for 8 p.m. Sunday, setting the stage for the biggest showdown of the fall television season--and not just between ABC and NBC. CBS is the reigning champion in that time slot with “Murder, She Wrote,” returning for its 10th season after finishing No. 5 overall in the prime-time ratings last season. And Fox joined the fray in a major way by moving in the half-hour comedy “Martin,” its top-rated new show of last season, when it was seen on Thursdays, then following it at 8:30 with a very compatible new comedy, “Living Single.”

“Welcome to the battle of the titans,” says Deborah Joy Levine, creator of “Lois & Clark.” It begins tonight.

The stakes in the battle are enormous. The hour between 8 and 9 on Sunday night is the most important in network television. The reason: More people watch TV on Sunday night than any other night of the week, and more of them watch during that hour than any other. Thus, a strong showing there sets up the program that follows--movies in the case of ABC, CBS and NBC--serves as a tremendous promotional platform to plug other network programs coming up that week and weighs heavily in the weekly ratings averages.

Last season, “Murder, She Wrote”--itself the beneficiary of a powerhouse lead-in from the top-rated “60 Minutes”--attracted an average of 25% of the people who were sitting in front of the TV on Sundays at 8 p.m., compared to 18% for ABC and 15% each for NBC and Fox. The “CBS Sunday Movie” then won the competition from 9 to 11 p.m.--drawing the same 25% of the audience--and CBS in turn won the prime-time ratings race.

“It’s been our experience in the last couple years that we are winning the week through Saturday night, and our final standing comes down to how well we do on Sunday night,” said Alan Sternfeld, senior vice president for program planning and scheduling at ABC. “In many instances, up until 7 on Sunday night we would be the leading network in ratings for the week, then CBS would be propelled to a win on the strength of ’60 Minutes,’ ‘Murder, She Wrote’ and movies.”

Peter Tortorici, senior vice president of programming for CBS, doesn’t dispute Sternfeld’s analysis. “They don’t pay off horse races on how you manage to cross the finish line,” he said. “They only pay off on the basis of whether you cross first.”

Dating back to the earliest days of television, Sunday night has been home to some of the medium’s most popular programs, from Ed Sullivan’s “Toast of the Town” to “Maverick,” “Bonanza,” “Lassie,” “Walt Disney’s Wonderful World of Color,” “Kojak” and “All in the Family.”

“I remember, growing up, what it was like to sit around the television Sunday nights with my parents and watch a good show,” said Levine of “Lois & Clark.” “That hasn’t happened for me, a member of the thirtysomething generation, for a long time.”

“There used to be such great Sunday-night shows, like ‘Voyage to the Bottom of the Sea,’ ” echoed Phil Segal, vice president of Spielberg’s Amblin Entertain ment. “They may have been sophomoric, but they entertained, and (the industry has) moved away from that.”

The networks largely stopped producing action-adventure series in recent years because they grew to be too expensive in an era of soaring production costs and shrinking audiences. So the action series, led by “Star Trek: The Next Generation,” migrated to first-run syndication, where they have further contributed to the erosion of network ratings by stealing away viewers.

Now, the networks are fighting to bring them back, and ABC and NBC have chosen Sunday night to roll out their class acts for the 1993-94 season. “Clearly, in terms of the one-hour form, there was a definite desire on the part of the networks to get back to much more action-adventure,” said Leslie Moonves, president of Warner Bros. Television, which is making the new “Superman” series. “So it was a natural that these two shows that are both bigger than life--’Lois & Clark’ and ‘seaQuest’--would see the light of day in a very important way.”

NBC was the first network to announce tonight as the launch date for its Sunday-night entry “seaQuest DSV,” a futuristic underwater thriller featuring Roy Scheider at the helm of a high-tech submarine. NBC ordered a full season’s worth of 22 hours of the series, to be produced at a huge price tag of $1.7 million per episode, in order to get into business with Spielberg, who has become hotter than ever with the success of “Jurassic Park.”

“Our goal,” said Preston Beckman, NBC’s vice president of program planning and scheduling, “is to become the ‘Bonanza’ of the ‘90s.”

ABC executives initially had planned to introduce “Lois & Clark” two weeks from tonight--because next Sunday the network must put its regularly scheduled programming on hold to televise the Emmy Awards--but they changed their minds to avoid letting “seaQuest” get a head start.

“This is a very competitive environment--the whole month of September--with the launching of new shows and the staggering of series premieres,” explained ABC’s Sternfeld. “No network likes to give another network a free pass. Viewers are sampling shows, forming habits of TV viewing that will take them through the season. Since we have the Emmys on the 19th, we decided not to do the other network the favor of giving it several unopposed broadcasts.”

“Lois & Clark,” starring Dean Cain as a gentler Man of Steel and Teri Hatcher as Lois Lane, has a 13-hour order at a cost of $1.4 million per episode. It’s described as a more romantic version of the superhero. “It was originally pitched to ABC as the ‘Moonlighting’ version of Superman,” said Warner Bros.’ Moonves.

Both series feature the extensive use of special effects. All of the underwater hardware in “sea-Quest” was digitally created on a computer, while the interior sets sprawl over five sound stages at Universal Studios. “Lois & Clark,” meanwhile, hired John Sheely, who supervised the special effects for the “Coneheads” movie, to use similar computer-generated imagery to illustrate Superman’s X-ray and telescopic vision and to make him fly.

Unwilling to give ground, CBS also changed its plans for “Murder, She Wrote,” moving its two-hour season premiere from Sept. 26 to tonight.

“If they’re ready to do battle, so are we,” Tortorici said. “We’re not going to give them an opportunity to come to our audience simply by not competing. It’s a very important time period, and we’re there to continue to give the audience that has been coming to us every reason to stay. So because the other networks were saying, ‘We have something new and exciting,’ it was important for us to say, ‘OK, we have something new and exciting but familiar to come back to.’ We want to be competitive from day one.”

Fox, on the other hand, decided to get competitive before day one. It launched “Martin” and “Living Single” three weeks ago in an effort to establish a foothold in the time period before the other networks’ big guns were brought in. The strategy appears to have worked: For those three weeks, the two comedies have won the night in ratings with adults 18 to 49.

Those numbers are expected to drop after tonight, but TV analysts expect Fox to do fine, especially among the young viewers it covets, because it is offering the only comedy for that hour.

“In general, we had the advantage of watching the other three networks announce their fall schedules prior to our announcement,” said Sandy Grushow, president of the Fox Entertainment Group. “The founding and guiding principal of this company has been that of an alternative. When we saw both ABC and NBC go in there with two new one-hour dramas, we realized that the greatest opportunity in the time period was with two comedies. It’s a classic case of counter-programming.”

As the defending champion, CBS is the odds-on favorite to win the Sunday-night shootout with “Murder, She Wrote.” Thanks to the star power of Angela Lansbury, the one-hour mystery has perhaps the largest following on television of viewers over 50, a demographic only CBS seriously pursues. And supervising producer Bruce Lansbury, Angela’s brother, says they are attempting this season to broaden the reach with younger guest stars and stories that might have more youthful appeal, such as one in which Jessica Fletcher turns to a 16-year-old computer whiz kid for help in solving a Silicon Valley murder.

Fox’s “Martin” and “Living Single” also seem to be on safe ground as the only option for people looking for comedy. That leaves “seaQuest” and “Lois & Clark” to duke it out.

“One way or another, somebody is probably going to score a quick knockout,” NBC’s Beckman predicted. “I’ll be honest, I don’t see how both shows can last for weeks and weeks.”

With NBC paying a hefty $1-million license fee to Universal for each episode of “seaQuest,” and ABC paying Warner Bros. almost as much for “Lois & Clark,” neither can afford to see their series languish in the ratings.

“Hopefully this would be an example of something good for network television in general, by increasing an already high network viewing time period,” Beckman said. “But the reality is, ‘Martin’ will make it tough for these two shows to coexist by the mere fact that Fox is there draining off some of that audience.

“My best guess is that viewers have been making up their minds for a long time, so that by the time the shows come on it will be all over. In a way, it’s been a battle of the promo guys.”

Levine, who also is executive producer of “Lois & Clark,” is worried by that prospect. She went to great lengths in the first installment to set up her unique vision of Superman--as a farm boy who, because he wanted to be a great writer, traveled all over the world before landing at the Daily Planet. His mom sews him a Superman suit only after he decides he needs a disguise.

Now she fears that everyone will tune into “seaQuest” tonight and that, if they later decide to watch “Lois & Clark,” they might not understand the completely different Superman premise she painstakingly laid out in the premiere.

“It’s regrettable in TV terms,” ABC’s Sternfeld admitted. “Members of the fourth estate take us to task for these megaton exchanges--my big program airs the exact same time as another network’s big program. That’s just as problematic for networks as viewers. But the truth is, unless you just want to put up color bars and run a test pattern for a couple hours and say, ‘OK, fine, we gave you a chance to sample the other show, now come back to us,’ there’s no other way to do it.”