NBC Top Network Last Season; ‘Roseanne’ Ties ‘Cosby’ as No. 1 Series : Television: Figures show a mysterious drop-off in number of homes tuning into Big 3.

NBC was the top network for the 1989-90 television season, it was a split decision between “The Cosby Show” and “Roseanne” for No. 1 series, and there was a mysterious drop-off in general television viewing, according to end-of-season figures released Tuesday.

The 30-week prime time season ended Sunday with NBC way out in front, posting its fifth consecutive win with a 14.6 rating and a 24 share, according to the A. C. Nielsen Co. ABC was second with a 12.9 rating and a 21 share, and CBS was third with a 12.2 and a 20 share.

In evening news, ABC’s “World News Tonight” with Peter Jennings was the new champ, with a 10.9 rating, with “CBS Evening News” with Dan Rather demoted to second with a 10.0 rating. “NBC Nightly News” with Tom Brokaw was third with a 9.9 rating.

The question of which show was on top of the list was a matter of whether or not you counted a “Roseanne” episode broadcast outside of its regular time period. NBC and ABC called the win for ABC’s “Roseanne,” while CBS gave it to NBC’s “The Cosby Show.”

“It’s no surprise,” said Bob Niles, NBC vice president of research, in announcing the season-end statistics at a news conference.

The three-network share of homes using television--HUT levels--was down from the 1988-89 figure of 67%.

David Poltrack, CBS senior vice president of planning and research called the decline “unprecedented and unexplained” and said CBS was pressing Nielsen for more data.

The top new program for the season that aired six or more times was ABC’s “America’s Funniest Home Videos,” which had a 20.9 rating and a 32 share.

“Twin Peaks” on ABC won as most-watched made-for-television movie. Top miniseries of two or more parts was ABC’s “Small Sacrifices.” The most-watched theatrical movie was “Lethal Weapon” on NBC. The Academy Awards on ABC won as top entertainment special.

Outside of prime time, CBS was the daytime winner, with “The Young and the Restless” topping the chart. In morning programming, ABC’s “Good Morning America” was top-rated, with NBC’s “Today” show falling off after Deborah Norville replaced Jane Pauley. CBS was third.

TOP 10 SHOWS The top 10 regular series for the 1989-90 season, according to the A. C. Nielsen Co., were:

1. Roseanne (tie): (ABC)

2. The Cosby Show: (NBC)

3. Cheers: (NBC)

4. A Different World: (NBC)

5. Funniest Home Videos: (ABC)

6. Golden Girls: (NBC)

7. 60 Minutes: (CBS)

8. Wonder Years: (ABC)

9. Empty Nest: (NBC)

10. Monday Night Football: (ABC)