VIDEOS : Biggest Season Ever Has Its Down Side Too : Sales: Retailers enjoy 24% increase over last year, but a glut of major titles is a rock in some company stockings.

For home-video fans, last month was the merriest December ever. For video retailers, it was fairly glorious too. And most video companies were happy because sales boomed during the holiday season. But for some video companies, because of a rental-market glut, it was a disappointing month.

It was the biggest holiday sales season ever for home video. According to a study by the New York research firm Alexander & Associates, sales were up 24% in the September to mid-December period to $620 million, compared to $470 million in a comparable time period last year.

Yet, because of the unusual glut of major new rental titles last month--including “When Harry Met Sally,” “Star Trek V,” “Great Balls of Fire,” “Nightmare on Elm Street V,” “Lock Up” and “Weekend at Bernie’s”--video companies sold fewer copies of these major titles than expected to retailers and distributors. So some manufacturers such as Paramount, which released “Star Trek V,” didn’t make as much money as predicted on these big movies.

December used to be a relatively dead month for new rentals. Video companies rarely shipped major titles in that month, figuring retailers had exhausted their budgets in October and November stocking up to meet the demands of the Christmas sales market.

Explaining the sudden change of strategy last month, Alan Perper, vice president of marketing for Paramount, said, “Many companies had the same idea at the same time: to put out a big rental title in December, which had traditionally been such a quiet rental month. This year there were eight to 10 major rental titles as opposed to one or two last year. We all figured that some of the big holiday titles like ‘Batman’ and ‘Bambi’ would increase traffic in the stores, which would increase rentals.”

Normally, he said, some companies might have waited until mid-winter to put out these rental titles. But they wanted to avoid the heavy competition from low-priced releases in February--”Lethal Weapon 2” and “Indiana Jones and the Last Crusade”--and March, when “Honey, I Shrunk the Kids” is due.

Referring to “Star Trek V,” Perper said: “We sold about 10% less (to retailers and distributors) than we had hoped to sell. Everybody did less on their big titles.”

But Nelson Entertainment, which released “When Harry Met Sally,” didn’t suffer, according to company president Reg Childs. Industry sources claim that the company expected to ship 400,000 copies but wound up putting only 300,000 in the marketplace. Childs insisted that the company shipped 363,000, which, he added, just about matched their expectations.

Though the rental-title glut hit some manufacturers right in the wallet, the flood of new movies in December was an added Christmas present for renters.

Allan Caplan, of the Midwest’s Applause Video chain, reported bang-up business for the week after Christmas.

“It’s a great week for rentals,” he said. “Schools and colleges are closed. Kids, teen-agers and college students are home looking for something to do. Christmas shopping is over so people can relax. They want to rent movies. That week they’ll rent anything. The week before Christmas isn’t so great because people are still busy getting ready for Christmas. But the week after, we did three times the business we did in the week before.”

The latest report on “Batman”--out in mid-November at $24.98--is that sales have slowed down but rentals are still thriving. It’s been No. 1 on the Billboard magazine rental chart since early December.

“Batman” sales have been strong too, but some retailers indicate that they are not as strong as expected. Apparently there’s no shortage of copies for sale. According to estimates, Warner Video shipped between 11 million and 12 million cassettes to retailers and distributors.

Tower Video executive John Thrasher said that his company’s stores have sold 40,000 copies of “Batman” but still have thousands of copies in stock.

But this is video retailers talking. Just because “Batman” is not sold out in video stores--which usually offer only small discounts--that doesn’t mean its sales are slumping. “Batman” is sold, often at huge discounts, in many other outlets.

The study by Alexander & Associates confirms that outlets other than video stores have been successfully selling titles like “Batman,” “Bambi” and “Who Framed Roger Rabbit”--which were the most popular movies in the home-video sales market this last holiday season. Video stores, the study reported, accounted for only 16% of holiday sales. Mass merchants--discount chains like Target--did 37% of the sales business.