A 35-year-old College Park woman pleaded guilty in federal court here yesterday to participating in a large-scale, four-year scheme in which she bought thousands fo weight-reducing pill with phony prescriptions to sell to customers on the street as popular psychic "downers" and "uppers."

Darlene Jenkins Bronz appeared before U.S. District Court James Joyce Hens Green and pleaded guilty to one count of conspiracy and another of making false statements to a grand jury investigating the scheme. She faces up to nine years in prison and $40,000 in fines. Bronz, who said she is unemployed, is scheduled to be sentenced on May 21.

Washington physician Isidore Shulman, a cardiologist specializing in weight control, was named by prosecutors as an uncharged coconspirator who wrote about 2,000 prescriptions for Bronz between September 1976 and December 1980. Prosecutors said possible collusion in the case by several pharmacists in the Washington area is still under investigation, but declined to give other details.

Shulman, now 72 years old and hospitalized with terminal cancer, has not been charged in the case, prosecutors said, because of the improbebility that he could appear in court to defend himself.

The Bronz plea marks the latest in a series of cases mounted by federal prosecutors and the D.C. police department's special drug diversion investigations unit to curb the lucrtive street trade here in prescription drugs sold for illicit and private, unsupersived, recreational or therapeutic use.

In yesterday's case, Assistant U.S. Attorey Douglas J. Behr said in a six-page criminal information filed against Bronz that Shulman issued prescriptions for nearly 82,000 pills and capsules -- most of them stimulants called Biphetamines and depressants called Quaaludes -- during the four-year period of the scheme, writing prescriptions in the names of numerous people, including some of his former patients.

Bronz, a heavy-set woman who had been a patient of Shulman herself for several years, paid the doctor at the rate of about $15 for two prescriptions for each "patient names" -- usually one stimulant prescription and one depressant prescription -- and then purchased the drugs at pharmacies in Washington and suburban Maryland, Behr said.

Statistics in the information indcate Shulman obtained an average of 1,000 stimulant pills and about 800 depressants a month. Drug enforcement agents estimate such pills sell in bulk quantities on the street here for about $2 each.

According to the information, Bronz visited Shulman's downtown Washington office about once a month and purchased the prescriptions. At times, the information said, Shulman postdated the prescriptions "to make it appear that they had been issued in the normal intervals" for weight-reducing treatment.

Henry F. Schuelke III, attorney for the ailing Shulman, said in a telephone interview yesterday that if the doctor were able to defend himself, "he'd demonstrate his complete lack of knowledge that [Bronz] was selling the stuff" on the street.

"He may have been a little naive," Schuelke said, "and he may have naively believed his patients used the drugs for the purpose for which they were prescribed."

As a further indication of the weakness of the case against Shulman, he said, the doctor sent away several would-be patients who were clearly not overweight and who turned out to be undercover police officers attempting to obtain pills.

Doctors specializing in weight reduction prescribe Biphetamine to curb patients' appetites. Because it is a stimulant, however, it can cuase sleeplessness and other advese side effects. Doctors thus also prescribe Quaalude, a depressant, to conteract the Biphetamine.

When diverted to illicit street use, Biphetamines -- called "speed" or "black beauties" -- are considered to be an emotional "upper," giving the user a heightened sense of well-being and endurance. Quaaludes, on the other hand, are considered "downers," often mixed with alcohol and reputed to "mellow some people out," as one law enforcement offical said yesterday.

Quaaludes are a favorite drug of middle-class housewives, he said. "You know, Mamma's little helper."