After 16 weeks of testimony and argument, the jury began deliberations yesterday in the federal bribery and conspiracy trial of former high-ranking District official David E. Rivers and contractor John B. Clyburn.

With more than 1,000 exhibits, including 300 conversations secretly taped by the FBI, the prosecution has mounted an intricate case against Rivers, the former head of the city's Department of Human Services, and Clyburn, his close friend and an influential contractor.

The jurors, having been instructed on the law by U.S. District Judge Joyce Hens Green, deliberated for just over an hour before retiring for the night.

The government has charged that Rivers and Clyburn illegally steered nearly a dozen city contracts worth more than $2 million to Clyburn's companies and those of his friends. Prosecutors have attempted to show that from 1983 to 1987, the two men manipulated contract procedures, corrupted other D.C. officials and mounted coverups to hide their misdeeds from the news media and public.

Rivers, they charged, steered contracts to a company set up by an FBI undercover agent in exchange for future benefits he would collect upon leaving the government.

The defense, meanwhile, has focused on testimony that Rivers never accepted any cash from the undercover agent, despite frequent opportunities.

And defense attorneys for both men have suggested that Rivers was merely trying to help minority contractors and Clyburn was doing savvy networking -- all well within the bounds of legality.

Clyburn's attorney, Thomas Dyson, argued that Clyburn simply used his government contacts to win business for his companies. "That's what people in Washington, D.C., do all the time," Dyson told the jurors last week.

Rivers's attorney, Francis D. Carter, said his client in effect told minority contractors: "There's the watering hole. Go to it . . . . I'm going to introduce you to people." But Carter concluded, "Criminally corrupt? Not by any means."

Yesterday, Assistant U.S. Attorney Rhonda C. Fields countered their arguments in her two-hour rebuttal -- the prosecutor's final word to the jury. Fields said that Rivers and Clyburn were, indeed, "trying to help some minorities. They wanted to help a particular, very specific minority, a minority of John Clyburn's friends, a minority of David Rivers's friends . . . a minority of those who wanted inside information," said Fields. "Not a minority out there scuffling to get business. They weren't concerned about ensuring a level playing field for them. They were concerned about their friends."

The trial focused on four sets of contracts providing drug counseling for schoolchildren, crisis services to the mentally ill, computer services, and analyses of the city's troubled drug treatment agency.

Much of the evidence in the five-count indictment was obtained by FBI agent R. Leonard Carroll, who posed as an Atlanta businessman and worked with Detroit businessman Warren Barge, who was unaware of the FBI operation.

The 300 secretly recorded conversations gave jurors an inside view of contracting in the District and served as the heart of the prosecution's case.

Rivers gave B&C Management, the company started by Barge and Carroll, a $24,995 contract to conduct an energy audit for the city after Carroll promised to contribute to a fund-raiser that Rivers was holding for Mayor Marion Barry, prosecutors charged.

The question of whether Rivers received any benefit for the transactions is a central issue in the case.

In a conversation taped in April 1987, Carroll told Rivers, who had moved from the Department of Human Services to become secretary of the District of Columbia, of a plan in which Rivers would arrange city contracts and Carroll would kick back contract money into a bank account for him. Rivers told him, "You got my green light 100 percent . . . . Go. Do it."

On cross examination, Carroll acknowledged that Rivers had never taken any cash from him.

"Dave Rivers never got one dollar bill from you, did he, Agent Carroll?" Carter asked.

"No, he did not," Carroll said.

As the trial moved into its final days, the judge sequestered the jurors after Rivers was mentioned in Barry's drug and perjury trial. A witness in that case testified that Rivers was one of several people who used drugs with Barry on one occasion. Rivers has not been charged with any crime relating to drugs.