Robert T. Howard president of NBC television since 1974, was fired yesterday on the eve of the 1977-78 season, expected to be most hotly competitive in television.

Robert E. Mulholland, a onetime Howard Protege, was named to replace Howard by Herbert S. Schlosser, president and chief executive officer of the National Broadcasting Company.

Mulholland has been moving rapidly up the corporate hierarchy in recent years and in March was named executive vice president for the Olympics.

He has a key role in landing the 1980 summer games in Moscow for the network, an $85-to-$100 million investment which NBC is gambling will become both a financial and artistic success, enhancing NBC's faltering image.

Although some network officials expressd surprise over the move, Schlosser reportedly has led it under consideration for six weeks and half offered the job to Mulholland about a week ago.

Under Howard's leadership, NBC had dipped to third place in the network ratings race last season after years of finishing second behind perennial leader CBS. ABC lapped both rivals to become the viewership champion in 1976-77, the first time ever.

Although last year' dismal finish undoubtedly contributed to Howard's ouster. Mulholland's rise to the top network post is only the latest in a series of top-level corporate moves recently made by Schiosser with the approval of NBC chairman Julian Goodman.

Mulholland's promotion yesterday coincided with an announcement that NBC Sports is being made a separate division, like NBC News, with Alvin Rush as the top sports executive.

Recently , executive vice president Mike Weinblatt has been given added authority in net work in as executive vice president for station relations.

The latter has become a troubled area as ABC has shown increasing aggressiveness in outbidding NBC for top affiliates in several important markets.

According to one NBC source, the timing of Howard's ouster, as the net-work gamblers to regain a challenging spot in the three-way rating race with ABC and CBS, was a "case of damaged if we do, demand if we don't."

"Herb has had the restructuring plans in mind for a long time, "the source said. "He wants to tighten the operation. If he waited to fire ratings it would look as if Bob were being made the scapegoat."

"Now, if we're successful. Bob will get some of the deserved credit, but meanwhile the long-range restructuring that was inevitable and probably would have cost Bob his job anyway, has moved forward." the source said.

Other industry sources suggest Mulholland is being groomed as Schlosser's eventual successor as NBC president. All three men are sometimes referred to as members of the "Hollywood Mafia" within NBC since all rose to prominence at the network after holding important posts in California.

NBC board chairman Goodman, a longtime news man himself, is known to be partial to news executives in top posts' another factor believed favoring Mulholland's eventual "coronation" as Schiosser's successor.

One less charitable, albeit knowledgeable source, said yesterday that "Herb's just preparing another scape-goat in case the ratings for the new season go bad for the second year in a row."

Ironically, Howard was vice president of KNBC TV, the NBC owned and-operated-station in Los Angeles until 1973. At that time, Mulholland answered to Howard as news director for the key station, and it was Howard who obtained substantial additional funding from the network for the news operation when protege Mulholland wanted to go to an early evening two-hour news show.

The ratings improved dramatically and both men subsequently moved to New York and executive positions as a reward.

Howard was unavailable for comment yesterday but network sources said the deposed president "hasn't decided what he's going to do in the near future."