Testimony at the trial of John W. Hinckley Jr. ended yesterday after Hinckley said he would not take the stand in his own defense against charges that he tried to assassinate President Reagan.

"It is totally my decision," Hinckley told Judge Barrington D. Parker when Parker asked whether Hinckley was acting on his own or abiding by a decision made by his lawyers.

Closing arguments from the prosecution and the defense are scheduled to begin this morning. The jury of seven women and five men is expected to begin its deliberations sometime Friday, after Parker instructs them about the law that applies to the case, including the key issues of the insanity defense.

Hinckley claims he was legally insane when he wounded Reagan and three others and should not be held criminally responsible for his actions. The jury must decide whether he was in control of his behavior when he fired on Reagan or whether he was mentally ill and as a result could not abide by the law or appreciate that his acts were wrong.

The final testimony in the case came from Dr. William T. Carpenter, an expert psychiatric witness for the defense, who was brought back to court to counter prosecution testimony that his interviews with Hinckley had planted ideas with Hinckley about mental illness.

Carpenter was the first expert psychiatric witness called by the defense a month ago. Later in the case, a prosecution psychiatrist, Dr. Parker Elliott Dietz, had sharply criticized Carpenter for his inexperience in dealing with crimnial defendants and said he inadvertently suggested things to Hinckley about his mental state.

Carpenter, director of the Maryland Psychiatric Research Center, testified that he was aware throughout his 45 hours of interviews with Hinckley that Hinckley might seize "on things that would be said or done in relationship to the insanity defense.

"I think it flies in the face of common sense to suggest that anyone would not have an awareness and concern about that," Carpenter told the jury in response to Dietz' criticism of him.

Carpenter said that any characterizations he put on Hinckley's thoughts were based on concepts raised by Hinckley or drawn from other sources and were intended to help him explore Hinckley's state of mind.

For example, Carpenter acknowleged that he had said to Hinckley during one session that it appeared Hinckley considered Reagan and his other victims as "bit players" in his goal to achieve a "fateful union" with actress Jodie Foster. Carpenter told the jury, however, that the remark was made after a long interview with Hinckley in which Hinckley had described Foster as a "major person in this," but had not shown "even an ordinary concern" for the consequences to his victims.

Carpenter said yesterday that during his interviews with Hinckley he learned that a confrontation Hinckley said he had with Foster while she was a student at Yale University had not in fact occurred.

During cross-examination by chief prosecutor Roger M. Adelman, Carpenter repeatedly refused however to characterize Hinckley's story as a lie, calling it instead a "fabrication." After some back and forth between Adelman and Carpenter, in which the psychiatrist said some fabrications are lies, Parker finally demanded: "Well, doctor, in this particular instance was it a lie?"

"It was not true," Carpenter testified. At that point, after a burst of laughter in the courtroom, Adelman ended his questioning.

Parker had agreed to allow Carpenter to testify over objections from the prosecution, which contended that the defense was simply trying to have the "last word" in the testimony.

Hinckley remained quietly in his seat during yesterday's proceeding. Late in the day, after the jury was dismissed and as Parker was about to begin to discuss jury instructions with the lawyers, Hinckley asked Parker, through his lawyers, if he could leave the courtroom.

Parker called Hinckley to the lectern and asked if he wished to be excused, to which Hinckley responded, "That's correct, sir."

Parker granted Hinckley's request after Hinckley added, "I'll be back tomorrow."