On the morning of March 30, 1981, after a long cross-country bus trip to Washington, John W. Hinckley Jr. left his room at the Park Central Hotel, went to a McDonald's restaurant and a Crown Books store, bought a copy of The Washington Star and read President Reagan's schedule for the day.

Back in his room, Hinckley took a shower, loaded a .22 caliber pistol with exploding bullets and wrote a letter to actress Jodie Foster saying, "I just cannot wait any longer to impress you." He put a John Lennon button in his pocket and took a cab to the Washington Hilton Hotel where he shot President Reagan, his press secretary, a Secret Service agent and a D.C. police officer.

Depending on whom you believe, these were the acts of either a disturbed but coldly calculating assassin or an insane man finally driven out of control by bizarre and violent delusions. Was he crazy, or just sick?

Experts testifying on Hinckley's behalf have given the jury a chilling picture of a man consumed by severe mental illness, possessed by an "inner rage" and tormented by thoughts of murder and suicide as he pursued an eternal union with Foster.

As the trial moves through its fourth week, however, the jury will begin to hear testimony from prosecution experts who have probed just as deeply into Hinckley's state of mind and arrived at sharply different conclusions. They will challenge the degree of Hinckley's mental illness, which the defense has portrayed as extreme and deep-rooted. They will contend he could have abided by the law, but chose not to.

In the end, the jury of 12 laymen will be left with a set of contradictory opinions about a single man who has been subjected to hundreds of hours of scrutiny by skilled specialists on the function--and secrets--of the human mind.

The jurors, through the testimony of three defense experts, already have heard the key questions that the law says could excuse John Hinckley from criminal responsibility on the grounds of legal insanity. Did he suffer from a mental disease or defect last year when he wounded President Reagan and three others? As a result of that mental disease or defect, was he unable to abide by the law, or unable to appreciate that his acts were wrong, or both?

So far in the trial, the defense experts have put varying, but equally severe, labels on the mental illness they testified they found in Hinckley. All the labels relate to deep depression and longstanding schizophrenia, an extreme break with reality in which the emotions dry up and the mind is dominated by delusions.

Yet, they say, the very nature of Hinckley's disease is such that he still could make his way through life. He could get good marks on college papers, he could rent apartments and make polite telephone calls, he could get money from his parents, he could start his own mail-order business--and he also could go target shooting and make frenzied airplane trips around the country pursuing Jodie Foster and stalking candidates for president.

Hinckley's rage was trapped in his "inner mind," they said. He was not "a madman dashing about looking like a monkey," said defense psychiatrist Dr. David Michael Bear.

Indeed, before them every day the jury sees a rather ordinary young man, wearing a crisp three-piece suit or a navy blazer, who shows none of the bizarre behavior that laymen might expect from a madman.

The only inappropriate thing Hinckley has done at the trial was to stalk out when he heard Jodie Foster--the unattainable object of his fantasies--say she did not know him. The jurors, watching Foster's taped testimony on television monitors, seemed almost not to notice his departure.

They saw him sit steely and cool through his parents' wrenching testimony and they saw him laugh and shake his head, with amusement or embarrassment, when some of his writings were read aloud. His lawyer, outside the hearing of the jury, has described Hinckley as a "very charged individual," but, most of the time, the jury sees only his distant stare.

When the prosecution begins this week to counter Hinckley's insanity defense, the first witnesses will not be their psychiatric experts but the ordinary people who saw Hinckley around the time of the shooting, from the chambermaid at the Park Central to the police officers who booked him at headquarters.

For starters, the jury will hear from former deputy D.C. medical examiner Dr. William J. Brownlee, who has said in a report that Hinckley did not look like a "classical nut" when he examined him hours after the shooting.

When the moment finally arrives for a verdict--still weeks away--the jurors will get only a bare-bones formula from the court to help them decide, deliberately leaving them to apply their commonsense notions about criminal responsibility to the framework of information provided by the psychiatric specialists.

Studies conducted in Washington have shown that jurors more often than not fail to understand instructions the court gives them in insanity cases and apply traditional ideas about knowing right from wrong that the courts abandoned long ago. Moreover, legal scholars believe that the ultimate question in the jury's mind when they sit down to decide Hinckley's case likely will be the consequences of their verdict.

At the close of the trial, it is expected that Judge Barrington D. Parker will tell the jurors that Hinckley will be confined to a mental hospital automatically if they find him not guilty by reason of insanity. They will also be told, however, that if that happens there will be a hearing within 50 days to determine if Hinckley is entitled to release because he is no longer a danger to himself or others because of mental illness.

If they find him guilty, he most likely would be sentenced to prison.

That, when the experts are through, will be the question for the jury: Should Hinckley be in the hospital or in jail for what he did?