For years, Jo Jo Brown was the king of "Bam" on the 600 block of S Street NW, a once-popular spot in the midst of Washington's illegal, yet booming curbside drug market.

On weekend nights, according to police, cars of drug users lined up to purchase a seemingly uninterrupted flow of Bam, the street name for Preludin, a pink diet pill commonly used by addicts to boost heroin highs.

Brown operated behind the scenes there for eight years, police said, relying on trusted lieutenants to make the actual street sales. City narcotics officials say he was well-known to them, but they were unable to make a case against him.

Brown's luck ran out last month when a U.S. District Court judge here sentenced him to up to 10 years in prison after he and four other men described by police as major suppliers pleaded guilty to charges relating to the sale of Preludin.

The five were arrested after a nine-month undercover investigation when police and federal drug agents staged a phony, cut-rate sale of Preludin at a motel on June 6, climaxing an investigation that relied heavily on secret tape recordings of the drug deals and a former Preludin-dealer-turned-informant who helped arrange the purchases.

Besides Brown, 41, the other four suspected major dealers are Ronald (Heavy) Hinton, 36; Clarence Watts Jr., 38; his brother, Thomas Watts, 37, and Marvin Lee Cobb, 30. They are now in prison.

The last one to be sentenced was Clarance Watts who appeared yesterday before senior Judge George L. Hart Jr., who also handled the other cases. The judge sentenced Hinton, the Watts brothers and Brown to up to 10 years in prison and ordered that they serve at least three years and four months before they become eligible for parole. Cobb was sentenced to up to five years in prison and must serve 20 months before he is eligibe for parole.

Prosecutors and police said yesterday that besides the successful prosecutions, the investigation also has helped to significantly reduce the amount of illicit Preludin now available in the District. As a result, the retail price of Preludin has shot up from $10 to $20 per pill, police said.

"It's very hard to find any," said narcotics Det. John Mathis, who helped lead the probe.

Prosecutors and police credit the probe's success to the unusually close cooperation between the U.S. Attorney's office, the police and federal drug agents, agencies that at times have feuded with each other.

Assistant U.S. Attorney George J. Terwilliger III worked closely with the investigators from the beginning and said he urged the investigators to tape record as many of the transactions as possible. The final sale in the motel room was videotaped.

"With the tape recordings, there could be no dispute about what the defendants had to say during most of the transactions," Terwilliger said.

As Thomas Watts rode to the motel room June 6, he was secretly recorded as saying: "I got some people who live way up there in Frederick, man, and like they buy six or 7,000 pills at a time," according to a police transcript of the recording.

Watts added later: "There ain't no place that move them pills the way they move them here in Washington."

A major break in the case came when John Henry Tate, a suspected Preludin dealer, agreed to cooperate with police after his arrest on Preludin possesion charges last August. Until then, police could not arrange undercover purchases from the five suspected dealers because they only sold to trusted associates like Tate.

"I knew what they the dealers were doing . . . but I just couldn't get somebody close" to them, said Det. Mathis.

With Tate's assistance, that no longer was a problem.

While Brown, whose full name is Joseph Lester Brown, ruled the Preludin sales at the S Street site, prosecutors said police successfully prosecuted several smaller dealers. Yet police were forced to turn to Brown for help when residents complained about drug trafficking on the block, prosecutors said.

Brown would almost immediately instruct his lieutenants to disperse and within a half hour the block would be deserted, prosecutors said.

Now, with Brown in prison, police say drug dealing has virtually stopped in the 600 block of S Street NW.