Sales executives at the major networks yesterday seemed generally unperturbed by reports that advertising agencies and Metromedia Television are trying to form a "fourth network" to give advertisers air time that is no longer available on ABC, CBS and NBC.

Under that plan, tentatively called "MetroNet," Metromedia and the agencies would originally underwrite 5 1/2 hours of new prime-time programming weekly and give them free to an informal "network" of as many of 25 independent stations in the major TV markets.

In turn, national advertisers would be able to purchase two minutes of commercial time in each half-hour of the new programs from the agencies. Such commercials couls reach as much as 50 per cent of the nation's 71.2 million TV households.

The key advertising target is the lucrative market of women aged 18 to 49, and the programming ammunition would include continuing Gothic serials and remakes of "Rebecca" and "Wuthering Heights." The programs would be mailed to stations participating in the "network" in time for identical air dates and times.

Sales executives at ABC and NBC yesterday had two basic criticisms of the MetroNet proposal, which is due to be explored further Friday in New York by major advertising agencies:

The 50 per cent of the national TV audience promised by the independent network is unlikely to satisfy advertisers accustomed to the 90 per cent and up delivered by the major networks.

A shortage of creative talent, already evident on the major networks, as well as a comparatively low programming budget, precludes development of competitive programming designed to attract big audiences away from established shows.

These major points were made last October by CBS network president Robert Wussler in a speech to advertisers on the subject of a fourth network, which is not a new idea in the advertising business.

At least four other attempts have been talked about in New York in the past six years, according to industry sources.

"The only people who make money out of this 'fourth network' talk," recalls a former top CBS executive, "are the guys who run the bars at Chasen's and "21'".

The pressure nevertheless has been building over the past several years as the big networks have enjoyed an unprecedented boom in advertising.

Major advertisers, seeking what they consider the largest possible market for their shrinking ad dollars, have been lining up for the mere 134 commercial minutes available in a week on any of the three big networks, each of which airs 22 hours of premium nightly programming in seven days. Each network permits six minutes of ads each hour in prime time.

This built-in limitation to available air time ("commercial inventory," the admen call it) has driven the price of an advertising minute to a record $6.50 a minute per 1,000 TV households reached.

Since the average Nielsen rating for a prime-time network show is a 20, which means an audience of 14.2 million homes, the current average commercial minute is something like $92,000.

Industry sources predict that when the prices are announced for the 1977 fourth quarter, starting in October, a hit like ABC's "Laverne and Shirley" will command as much as $150,000 a minute.

Network officials admit that all three networks have informally discussed expanding the allocated six minutes to seven an hour (as is done during presidential campaigns) out of the total of 9 1/2 minutes that the National Association of broadcasters TV Code permits.

But, they insist, not only would the extra minute have to come from valuable time now alloted to promotion of upcoming programs, but viewer reaction to two additional 30-second spots an hour would undoubtedly be unfavorable.

"Besides," said an ABC executive, "the relief would not be long-term to the advertiser. Frankly, I think the marketplace is going to take care of the problem, eventually. We just happen to be in a boom time right now."

Other sales executives aren't so sure. They point out that when the current recession first set in, the networks enjoyed their biggest year ever simply because with shrinking ad budgets, most advertisers felt TV was the single medium able to reach mass audiences.

An official of Benton and Bowles, an advertising agency also interested in the MetroNet concept, is trying to develop a second alternative. The firm proposes production of single, big-budget programs which would be actually "networked," or interconnected on telephone lines or via satellite, on a given night over selected independent stations. MCA-Universal has a similar project in the works.