Two Washington area men, described by law enforcement officials as long-time major suppliers of amphetamines to street users, pleaded guilty in U.S. District Court yesterday to drug distribution.

Joseph L. Brown, 41, of Wshington, and Marvin Lee Cobb, 30, of Alexandria, were arrested last month along with three other men after undercover District police officers and federal drug enforcemnt agents staged a phony, cut-rate sale of thousands of Preludin pills, known on the street as Bam and used as a heroin booster.

The arrest climaxed a nine-month investigation by the U.S. Attorney's office and law enforcement officials into illegal trafficking of the drug in Washington fro sources in other cities.

Senior Judge George L.Hart Jr. accepted guilty pleas yesterday from Brown to two counts of distribution of a controlled substance to an undercover police officer, once in a car and the second time in a Washington supper club on 11th Street NW. Brown, who is free on a $25,000 bond, also pleded guilty to one count of use of the telephone to facilitate a felony drug violation. He faces five years in jail and a $15,000 fine on each of the drug distribution charges and four years and $30,000 on the telephone incident.

Cobb, who is also free on a $25,000 bond, pleaded guilty before Hart to one count of drug distribution, involving the sale with Brown at the supper club.

Hart ordered presentence reports on both men and did not set a date for sentencing.

In a related development, Arthur Connelly of Philadelphia pleaded guilty last week in federal court here to two counts of drug distribution in connection with a sale of more than 800 Preludin pills to an undercover agent for the Drug Enforcement Administration.

Law enforcement sources said that Connelly, who made the sale in a hotel room on New York Avenue, had been anoriginal target of the investigation. Connelly voluntarily appeared in federal court here after he was arrested in Phildelphia in June.