As the government prepares to rest its case this week in the trial of D.C. Mayor Marion Barry, the defendant faces a decision that could mean the difference between freedom and federal prison: whether to testify or remain a silent spectator while his lawyers argue that the prosecution's case is flawed.

For Barry, the decision is charged with emotion, close associates say. Although the mayor has appeared calm and often jocular in the courtroom, friends say there are signs of great strain beneath the facade. Many days, they say, Barry goes home mentally and physically exhausted from watching the parade of witnesses state his many alleged misdeeds.

The mayor's lawyer, R. Kenneth Mundy, said his client is "chomping at the bit" to testify. Another source, familiar with the defense strategy, also said the mayor is eager to testify. Mundy also has said that a decision about Barry's testifying has been made, but he declined to reveal it.

On Friday, Mundy joked that Barry would make a "great" final defense witness. Sources said Mundy has cautioned Barry that the disadvantages of testifying probably outweigh the advantages. But Mundy has left the final decision to Barry.

One source close to Barry said he thought it "extremely doubtful" the mayor would take the stand.

"We've just got to hope that the jury gets the message that this case never would have happened if he weren't the mayor, and they don't need to hear him testify to figure that out," the source said.

The dilemma for Barry is this basic principle of criminal work: Juries almost always want to hear from defendants. When a defendant decides not to testify, a trial judge routinely admonishes jurors not to hold it against the defense -- and many jurors ignore those instructions.

Even so, it contradicts traditional legal wisdom for the mayor or Mundy even to entertain the idea of Barry's testifying. When a defendant testifies, he often becomes the prosecution's best witness, many trial lawyers and law professors maintain.

Charles J. Ogletree Jr., a Harvard law professor and former deputy director of the Public Defender Service here, said Barry absolutely should not testify.

"On a scale of 1 to 100, the likelihood of Barry testifying is zero, minus 15,000. There is no chance," Ogletree said. "He has nothing to add to the defense that's been developed by Ken Mundy, and it would be impossible for him to begin to respond to the direct evidence, circumstantial evidence and innuendo that is so pervasive in this trial.

"The only thing he could get out of his mouth without getting into trouble would be the words 'I do' after they administer the oath," Ogletree said.

According to sources close to the mayor, Barry has discussed with his lawyers the possibility of taking the stand to admit to virtually every allegation of drug use with Hazel Diane "Rasheeda" Moore, Lloyd N. Moore Jr., A. Jeffrey Mitchell, Darrell Sabbs and Hassan H. Mohammadi, as well as with Bettye Smith and Doris Crenshaw, who are expected to testify tomorrow.

The focus of his testimony, at least under direct examination by Mundy, likely would be to deny that he had used cocaine with Charles Lewis in Washington or the Virgin Islands, and to claim that he had no absolute knowledge Lewis was involved in cocaine.

Such a defense tactic could cause cross-examination problems for prosecutors, who would have lost much of their ammunition. And it could suggest to jurors the basis for a possible compromise verdict: finding Barry guilty of drug possession but not of lying to a grand jury about Lewis's involvement with drugs.

The problem with testifying is that only rarely can a witness exercise that type of control over a cross-examination. More to the point, the admission of regular, continuous drug use could make it difficult for jurors to acquit him on the perjury charges.

Furthermore, if Barry takes the stand, he must have a believable explanation for every material -- and immaterial -- allegation against him.

According to a former prosecutor, it is extraordinarily difficult for a public official to win an acquittal if he declines to take the stand.

"Anytime he does that, he's dead. He's gone. Goodbye," the former prosecutor said. "But in this case, I can't believe he would . . . . No matter how forceful you can be saying no, 16, 17 no's in a row just ain't going to cut it."

Assistant U.S. Attorney Richard W. Roberts is preparing to cross-examine Barry if he takes the stand, and Roberts has spent many hours during recent weeks framing questions that cover the entire range of the government's evidence, sources said.

The mayor could expect questions, for example, about why he attempted to place almost two dozen calls to Lydia Pearson, the owner of a Northwest Washington crack house, over three months. Why did Barry encourage Charles Lewis to go to the police after Lewis allegedly told Barry he intended to lie about what really occurred at a D.C. Ramada Inn in December 1988? And why did Barry tell Rasheeda Moore that she could expect no more help from him with her city contract unless she had sex with him?

Even if Barry answers convincingly, Roberts is sure to ask about each instance of alleged drug use mentioned by prosecution witnesses. Several possible defenses often used by drug defendants probably are not available to Barry, given the circumstances: alibis placing him elsewhere for each instance, multiple mistakes in ownership of the drugs and witnesses mistakenly identifying him.

The mayor probably would either have to deny he possessed drugs or, in effect, plead guilty to the various charges of possession.

Finally, any testimony by Barry could backfire. U.S. District Judge Thomas Penfield Jackson would be required to give a prison sentence only on the three perjury counts.

But he still could send Barry to prison if he is convicted on any of the 14 charges against him. Jackson would be able to consider all of the evidence presented at Barry's trial, and if he decided Barry had been untruthful on the stand, it could lead to a more severe sentence.