Several generic-drug companies are racing to enter the Valium market because the patent on the widely used tranquilizer expired last month.

Valium, sold by Hoffmann-La Roche Inc., has been one of the top-selling U.S. prescription drugs for the last decade. Sales last year of the antidepressant were $270 million, which represented about 25 million new and refill prescriptions.

The rush to move into the Valium market comes in the wake of a new generic-drug law designed to speed up and simplify the certification process. The law went into effect last November. The law created a drug-approval system for companies that want to copy brand-name drugs such as Valium, which were patented since 1962.

"There are a number of people who want to make this drug quickly," said William Grigg, a spokesman for the Food and Drug Administration.

The FDA must give its approval before anyone else can sell diazepam, the active chemical in Valium. Grigg said the agency was prohibited from disclosing how many and which companies are seeking FDA approval.

"There are probably about five or six companies that have petitions pending before the Food and Drug Administration," said Dee Fensterer, director of the Generic Pharmaceutical Industry Association in New York.

Barr Laboratories Inc., generic-drug manufacturer in North Vale, N.J., with 1984 sales of $20 million is one of the companies that said it has applied to the FDA to manufacture diazepam. "It's such a high-volume prescription drug that there should be a sizable market for generic versions," said Kathleen McGee, a spokeswoman for Barr. "But there should also be a lot of competition."

Par Pharmaceutical Inc., a generic-drug maker in Upper Saddle River, N.J., with 1984 sales of $17 million, also has an application pending. Zenith Laboratories Inc. and Mylan Laboratories Inc., two companies that Hoffmann-La Roche has sued for patent infringement, also might market diazepam, said John Doorley, a spokesman for Hoffmann-La Roche.

Lederle Laboratories, a Wayne, N.J., pharmaceutical manufacturer that is a subsidiary of American Cyanamid Co., also is trying to get approval from the FDA to market the drug.

"As a result of the new generic-drug law, I would expect more brand-name companies to get into the generic business as American Cyanamid has done with Lederle Laboratories," said Bill Corr, counsel to the House subcommittee on health and the environment.

Before the generic bill was passed in September, manufacturers of generic drugs had trouble getting FDA clearance for production of prescription drugs that have entered the market since 1962. Generic copies of drugs introduced before 1962 could win FDA approval without complete retesting of the generic copies. Before 1962, the manufacturer had to show only that the generic drug was the same as the pioneer drug to win approval as safe and effective.

The generic-drug market was a $4 billion industry last year, according to the Generic Pharmaceutical Manufacturers Association. Currently, about 40 percent of the top-selling prescription drugs have generic copies. By the end of the decade, nearly 100 percent of the 50 top-selling drugs will be free from patent restrictions and possibly available in generic form, the association said.

Generic-drug companies have to submit applications to the FDA and show that their product is bio-equivalent. Decisions on applications can take up to 180 days.

Grigg of the FDA said Hoffmann-La Roche is trying to delay its rivals. The company filed two petitions last month objecting to FDA's guidelines for determining whether the generic drugs are the bioequivalent of Valium.

Hoffmann-La Roche said it was asking the FDA for additional testing of the drugs because a study by the company comparing generic diazepams to Valium found that the other drugs didn't work as effectively as Valium.

"We believe the generic drugs, which weren't as effective as Valium, would have passed the current FDA guidelines for bioequivalency of generic drugs and would have been accepted for marketing," said Doorley of Hoffmann-La Roche.

Valium was developed by Leo Sternbach of Hoffmann-La Roche in Nutley, N.J. The drug had distinct advantages over other chemicals used to treat anxiety and it became a major commercial success. It was the No. 1 prescription drug in 1975, with sales of more than $200 million, representing 60 million prescriptions. Its use has dropped since then because of new competition and adverse publicity.

The drug has been criticized in recent years as being overused, and a 1975 study suggested a link between Valium taken by pregnant women and newborns with cleft palates. A study two years ago by the Boston University College of Medicine, however, found that there was no link, Hoffmann-La Roche said.