The Justice Department is investigating possible antitrust violations in agreements between record companies and the popular Music Television cable-TV network that give MTV exclusive rights to videos by major musical performers.

The investigation, which a department spokesman said yesterday has been under way for about a month, parallels a suit filed in federal court in Los Angeles last month by Discovery Music Network, a new cable channel similar to MTV that plans to go on the air Jan. 1.

The Discovery suit charges MTV and the record companies with violating antitrust laws by entering into the exclusivity agreements.

"We're just starting to take a look, and we haven't reached any conclusions yet," the Justice Department spokesman said. The spokesman could not say when the investigation might be completed.

MTV denied any wrongdoing. "We are confident that our agreement with the record companies does not violate the antitrust laws," the company said.

MTV was started four years ago by Warner-Amex Cable, the joint venture between Warner Communications Inc. and American Express Co. It was spun off as a separate company earlier this year.

MTV plays rock-music video clips 24 hours a day. It has proven to be an extremely successful venture, popular with advertisers seeking the affluent, primarily teen-age market the network reaches. The record labels credit the promotion provided by exposure on MTV with contributing to the recent resurgence in record sales.

Earlier this year, MTV began signing exclusivity pacts with major record companies, including Columbia, RCA, MCA and Polygram. Although details of the agreements have not been made public, it is generally believed that, in exchange for payments of cash and advertising, MTV is given its pick of the videos made by the labels' most popular artists, with a guarantee of exclusive use for at least 30 days. After 30 days, the videos can be shown on a limited basis on some music-video shows, such as NBC's Friday Night Videos, but they cannot be made available to competing music television networks for six months to a year.

There are no competing channels. But Discovery is set to go on the air Jan. 1, and Turner Broadcasting Co. will start a music channel this month.

Dain Eric, director of programming for Discovery, says his company filed suit to challenge the exclusivity deals because "we felt that MTV was trying to lock out any full-time competition. . . . MTV saw full-time competition coming, and they wanted to prevent that."

"Seeing as how the average life expectancy of a hit record -- and therefore for a hit video -- is two to four months, those videos don't do us any good after a year," Eric said.

By limiting exposure of their new songs to one network, record labels are being "rather shortsighted. . . . The name of the game . . . is, the more exposure, the more records they sell," Eric said.

But MTV said it believes the exclusivity provisions "are appropriate and valid, exclusivity being a common and accepted feature of entertainment industry contracts."