John W. Hinckley Jr. was flown to St. Elizabeths Hospital last night several hours after U.S. District Judge Barrington Parker ordered him committed to the federal mental institution in Southeast Washington.

Parker's order came one day after a jury found Hinckley was legally insane when he shot President Reagan and three others in March 1981.

Hinckley arrived at the hospital about 7:35 p.m., according to a spokeman for the U.S. marshals. The helicopter landed within the 20-foot walls of the hospital's maximum-security division for the criminally insane, called the John Howard Pavilion.

Hinckley will be housed in a private room in the six-story, brick-and-barred facility.

At St. Elizabeths Hospital, Hinckley will find himself in a secluded world with up-to-date facilities, highly regarded therapeutic programs and -- at least by comparison with any prison he might have been sent to if convicted -- a relaxed atmosphere.

There are about 240 inmates in the facility, many--like Hinckley--judged not guilty by reason of insanity (called "NGI" at St. Elizabeths), along with others judged mentally incompetent to stand trial, under observation pending trial, or who became mentally ill while serving prison terms.

The average stay at the pavilion for a criminally insane inmate -- for crimes ranging from sexual exhibition to murder -- is 4 1/2 years, according to hospital spokesman Dr. Harold Thomas. Hinckley has indicated he will not seek immediate release.

"Life will be very different than had he been in a prison," said Stanton E. Samenow, a clinical psychologist now in private practice in Northern Virginia, who participated in a decade-long study of the criminally insane at the pavilion, named after an 18th-century English prison reformer.

Samenow called the pavilion "a country club compared to a prison. The atmosphere is not as oppressive and conditions are better. . . . It's a hospital. You could say it's a permissive prison."

The pavilion, constructed in 1959, has tiled walls and floors, a modern kitchen, a gymnasium and outdoor recreational facilities, and complete occupational therapy facilities, according to Samenow. Most of the patients are in dormitories, each housing 10 to 12. There are organized sports teams and other recreational activities. Psychologists and medical aides are on constant call.

While security in the pavilion is tight, with locked doors, television monitoring and a high wall around the exercise yard, the pace of life is as close to normal as possible while staff members work with the patients -- no one is officially called a criminal there -- with the goal being to return them to the outside world.

Technically, Hinckley could be released in 50 days when Judge Parker must hold a hearing to decide whether he should be released because he is no longer a danger to himself or the community due to mental illness. But Hinckley's lawyers said yesterday he has "no current intention" of seeking release, and they said they will not represent him in an attempt to leave the institution until they are satisfied he is no longer a danger to himself or others.

F.J. Pepper, an Alexandria psychiatrist who once headed the maximum-security wards at John Howard where Hinckley will be held, said the St. Elizabeths staff typically has 40 days "in which to evaluate the case and submit a report to the court" for use in the initial release hearing.

During that time, Pepper said, Hinckley will initially be given a physical examination and then have daily conferences with a variety of people, including a social worker, a psychologist, a psychiatrist and others. He said it is "quite likely" that Hinckley will be given a psychological test.

"You can be assured that someone like John W. Hinckley will get a lot of attention," said Pepper, who worked at John Howard from 1971 to 1976 and now frequently examines criminal suspects for defense lawyers and prosecutors in Washington-area criminal cases.

Pepper said that staff doctors will be able to read the voluminous examinations that have already been compiled on Hinckley, although they are entitled to reach their own conclusions. Eventually, he said, those who have examined Hinckley will confer on the case.

They likely will talk en masse to Hinckley, Pepper said, then excuse him from the gathering and "see if a consensus exists, a consensus on whether he's got a mental disorder and whether he's dangerous" to himself or others.

Pepper said the staff could recommend to Judge Parker that Hinckley be released unconditionally, released conditionally or held for further treatment. He said immediate and unconditional release of a suspect found not guilty by reason of insanity is unlikely, "but it happens."

After the first hearing before Parker in 50 days, Hinckley would be entitled to a review of his mental state every six months.

Pepper said the hospital staff has the right, without court review, to move a patient from maximum security to minimum security within John Howard, but not to grant an unconditional or conditional release without a judge's approval. He said that a patient held in the minimum security section at John Howard has access to the campus-like grounds at the sprawling institution perched above the Anacostia River.

Both maximum- and minimum-security patients have access to the walled playing fields and the pavilion's gymnasium, he said.

Still, as one social worker familiar with St. Elizabeths said, "For a mental hospital, it's incredibly tight security.