John W. Hinckley Jr.'s actions before his attack on President Reagan were "not the reasonable acts of a completely rational individual," but he was not legally insane, government psychiatrists concluded in a report on his mental state.

One of those psychiatrists, Dr. Park Elliott Dietz, testified at Hinckley's trial yesterday that while the prosecution team found that Hinckley suffered a mood disturbance and personality disorders, there was no evidence of a serious mental impairment that would excuse him from criminal responsibility for the attack.

Dietz, an assistant professor of psychiatry at the Harvard Medical School, agreed with defense lawyer Vincent J. Fuller that the psychiatrists concluded that "Mr. Hinckley's history is clearly indicative of a person who did not function in a usual or reasonable fashion."

But the psychiatrist, who completed 4 1/2 days of testimony yesterday, said that on the day of the shooting, and earlier, there were "options available to Mr. Hinckley that did not involve shooting the president." Those options, Dietz testified, included Hinckley's own suicide or plans to kill himself in front of actress Jodie Foster, who had been the focus of Hinckley's obsession in the months before the shooting.

Dietz agreed with Fuller that the team of government psychiatrists said in their report that Hinckley "consciously" decided against any of those alternatives and chose instead to try to assassinate the president.

The defense, which is trying to convince the jury that Hinckley was legally insane when he wounded Reagan and three others, has contended through its expert psychiatric testimony that Hinckley was driven by an "inner world" of murderous and suicidal thoughts that left no room for rational decision-making.

The jury, which has now heard six weeks of testimony, will eventually decide whether Hinckley was mentally ill when he shot Reagan and whether, as a result of that illness, he was unable to abide by the law or appreciate that his acts were wrong.

Defense and prosecution expert witnesses have contradicted each other on those key issues.

Under questioning from chief prosecutor Roger M. Adelman, Dietz testified that, while Hinckley was frequently "thinking" of Foster, he might not have had an "obsession" such that thoughts about her intruded into his mind unexpectedly.

"My evaluation is he has been preoccupied with a number of things including fame, assassinations and Jodie Foster, and this is not a sign of serious mental illness," Dietz testified.

Dietz said that Hinckley's grandiose notions of his own self-importance were seen in his thoughts that he could win Foster's attention or affections or could become a famous songwriter or author.

Hinckley had ideas about becoming a famous criminal before the instant he became one when he shot the president, Dietz testified. Hinckley's idea that he was "special" and that he could become famous without work included his interest in committing a "high-publicity crime," Dietz told the jury.

Hinckley, seated at the defense table, shook his head.

In general, Hinckley, who had spent much of Wednesday mugging and posing for news reporters and artists, appeared more reserved yesterday.

Late in the afternoon, Hinckley, who had spent all day wearing a navy blue blazer, emerged from the cellblock in shirtsleeves.

After some words with his defense lawyers, Hinckley marched back to the cellblock, followed by an entourage of deputy marshals and came out wearing his jacket--and a sheepish grin.

The start of yesterday's session was delayed for 90 minutes without explanation.

Sources said later that the slowdown involved a matter with a juror, but the court transcript relating to the matter was sealed.

Late yesterday afternoon, the government called to the stand its second psychiatric expert, Dr. Sally Johnson, a staff psychiatrist at the federal correctional institution at Butner, N.C. Johnson conducted 55 interviews with Hinckley while he was there.

The court session adjourned for the day before Johnson's testimony went beyond a recitation of her academic background and her experience with criminal defendants. She will resume her testimony today.

Meanwhile, defense lawyer Fuller suggested through his cross-examination of Dietz that the government psychiatrists' decision to strictly adhere to diagnostic criteria in a handbook on psychiatric disorders resulted in their disregarding other signs that Hinckley had a serious mental disorder.

For example, Dietz agreed that one reason the prosecution team decided Hinckley suffered from a mood disturbance--rather than a more serious major depressive episode--was that Hinckley failed to meet the book's criteria which require that four symptoms of a major depressive episode be present every day for at least two weeks.

Dietz has maintained that the handbook, called the "Diagnostic and Statistical Manual of Mental Disorders," is a tool that allows psychiatrists to "speak a common language." He said its criteria are designed to provide a reliable diagnosis of mental disorders.

The prosecution has emphasized in questioning throughout the trial that defense psychiatrists' diagnoses have not fit into the categories set out in the book.