John W. Hinckley Jr., recovered from an apparent attempt to kill himself, was returned yesterday to St. Elizabeths Hospital, where he was placed under a 24 hour-a-day "suicide watch," according to a hospital spokesman.

Hinckley, who was found not guilty by reason of insanity last year in the March 1981 shooting of President Reagan and three others, was found Sunday morning semiconscious on the floor of his room at St. Elizabeths and rushed by ambulance to Greater Southeast Community Hospital for treatment.

St. Elizabeths spokesman Wayne L. Pines said Hinckley was in good condition yesterday, able to walk on his own and allowed to eat solid foods. Hinckley was returned to the same room he had before the incident, Pines said.

Hinckley apparently hoarded his own medication and then took an overdose of it and possibly other substances, according to knowledgeable sources. Pines would not comment on the incident other than to say that the hospital's investigation is not finished.

"A major part of the investigation is to discuss the incident with him, and that hasn't happened," Pines said. Officials at Greater Southeast hospital also declined to reveal the results of a laboratory report on the substances Hinckley took.

Pines said that Hinckley had been under "close surveillance" at St. Elizabeths before the incident, meaning that hospital personnel regularly checked his condition.

Hinckley will now be under "constant surveillance," Pines said, which means a registered nurse will watch him 24 hours a day both on the ward and through a window in the door to his private room there.

"We don't know how long the new procedure will last," Pines said, adding that that would be determined by Hinckley's therapist.

Pines said he did not know how much the extra care would cost, but he said he did not believe additional personnel would be hired. It now costs about $60,000 a year to keep a patient in the St. Elizabeths maximum-security section, Pines said.

Hinckley, who had been in serious condition and placed on a respirator to assist his breathing when he arrived at Greater Southeast hospital, was accompanied by deputy U.S. marshals on the ambulance ride back to St. Elizabeths.