Government prosecutors, in a surprise decision, declined yesterday to present any witnesses to rebut Mary Treadwell's defense against fraud and conspiracy charges, clearing the way for closing arguments and the start of jury deliberations.

The prosecutors' decision will leave the federal court jury of eight women and four men with a choice of whether to believe the testimony of Treadwell or that of her chief accuser, Zellene Laney, the former bookkeeper of P.I. Properties Inc., the now-defunct real estate firm that Treadwell headed in the mid-1970s.

Treadwell is accused of conspiring to use P.I. Properties to defraud the federal government and the impoverished tenants of the dilapidated Clifton Terrace apartment complex of thousands of dollars to enrich herself and her other business ventures.

Laney testified that Treadwell was an active participant in the alleged scheme, while Treadwell, during 13 hours of testimony last week, firmly denied that she illegally profited from P.I. Properties' ownership of Clifton Terrace.

Assistant U.S. Attorney Stephen R. Spivack declined to comment on why the government did not present any testimony to rebut Treadwell's staunch denial of criminal wrongdoing.

"Why is the sky blue?" the prosecutor asked a reporter. Spivack said it was "up to the jury to decide" whether the government had proved its case without any rebuttal witnesses.

Although the government is trying to prove that Treadwell engaged in a conspiracy, the prosecutors did not call as witnesses either of the convicted coconspirators in the case, Robert E. Lee, P.I. Properties' former general manager, and Treadwell's sister, Joan M. Booth, who was Clifton Terrace's project manager.

Treadwell, who fired Lee from his P.I. Properties' job in August 1977, and her court-appointed lawyers have repeatedly tried to deflect blame for any criminality that occurred during P.I. Properties' ownership of Clifton Terrace away from Treadwell and onto Lee and Booth.

In addition, the defense has sought to show that the Department of Housing and Urban Development, which sold Clifton Terrace to P.I. Properties in 1975 and foreclosed on the firm's largely unpaid mortgage in 1978, failed to provide enough money to make the project a success.

The self-confident Treadwell, 42, said during a break in yesterday's proceedings that prosecutors "had four folk they could have brought in here to say, 'She did it.' They didn't do it."

In addition to Lee and Booth, she was referring to two others who were originally charged with conspiracy in the case, Ronald S. Williams, the most recent of Treadwell's three former husbands and a man who worked on an audit of P.I. Properties' books in 1976 but never finished it, and Charles W. Rinker Jr., P.I. Properties' former secretary-treasurer.

Williams pleaded guilty to giving false statements to prosecutors in the case and was fined $5,000 and placed on three years' probation.

Rinker, who was called as a defense witness, was cleared of any wrongdoing in the case after he passed a lie-detector test professing his innocence. He testified that Booth, in a chance conversation he had with her last year, told him "that Mary and I were the only innocent parties in the case."

While Treadwell appeared to be buoyed by the fact that the government did not present any rebuttal testimony, her spirits seemed to sink a bit yesterday when U.S. District Judge John Garrett Penn denied a motion by one of her lawyers, John W. Nields, to dismiss the 21 charges she faces without sending the case to the jury. In addition to fraud and conspiracy, she is charged with tax evasion and making false statements to federal officials.

"There is a real danger the jury is not going to be able to understand" the technical intricacies of the various charges, Nields told Penn in asking him to acquit his client.

The defense lawyer argued that "the government is seeking to have the jury draw the most extraordinary inferences" in the case, chiefly that Treadwell, because she headed P.I. Properties, should have known all of what her subordinates were doing.

But Spivack argued that "this case should clearly go to the jury," and Penn agreed.