Psychiatrists at St. Elizabeths Hospital have determined that restrictions on mail sent and received by presidential assailant John W. Hinckley Jr. should be removed "in view of improvement in his clinical condition and judgment," according to papers filed in U.S. District Court here.

The decision, made Sept. 14 by a hospital review board at the urging of doctors treating Hinckley, came about three weeks after Hinckley filed a request with District Judge Barrington Parker for broader privileges during his confinement at St. Elizabeths.

Besides unmonitored mail, Hinckley has asked that he be allowed to place and receive unscreened telephone calls, be given access to reporters and be allowed to walk the hospital grounds for one hour each day, accompanied by hospital personnel.

It could not be learned immediately whether the mail restriction has, in fact, been lifted. Several calls to a hospital spokesman yesterday were not returned and lawyers in the case declined to comment.

The review board's action was disclosed in court papers filed by federal prosecutors in anticipation of a hearing scheduled for today before Parker at which Hinckley has asked to represent himself in seeking looser restrictions.

The government has said it opposes both the broader privileges and Hinckley's request to speak in court on his own behalf, including a chance to cross-examine hospital officials.

Hinckley told Judge Parker in a letter in July that he was considering dismissing two attorneys who represented him during his trial. The lawyers are expected to be present at today's hearing. Judge Parker has not indicated whether he will allow Hinckley to represent himself.

Hinckley, who shot and wounded President Reagan and three other men on March 31, 1981, has been confined at St. Elizabeths since June 1982 after a jury found him innocent by reason of insanity.

Evidence of improvement in Hinckley's condition raises a sensitive issue because, under D.C. law, he is entitled to petition Judge Parker every six months for his release. No such request is pending and the question of Hinckley's possible release is not expected to be a direct issue in today's hearing.

During a hearing before Parker in July, courtroom observers said Hinckley talked more coherently and appeared to be more in command of himself than during his 2-month-long trial, which was marked by erratic behavior.

Under previous restrictions, hospital personnel screened Hinckley's incoming and outgoing mail and were permitted to censor it, according to the court papers.

In an affidavit, Hinckley's psychiatrist, Dr. Joan A. Turkus, said the recommendation to lift the restriction was "based, in part, on Mr. Hinckley's agreement to continue, voluntarily, to show his correspondence with the media to myself . . . prior to sending it."

She said Hinckley also agreed "to share with us any incoming mail of a threatening or dangerous nature."

Turkus said the hospital review board decided to keep a log of the names and addresses of persons Hinckley corresponds with "in order to monitor the situation for the first three months after the restrictions are removed."

She recommended against removing other restrictions on Hinckley.

In the court papers, prosecutors said they oppose Hinckley's motion to represent himself because he has been judged mentally ill. Defendants who exhibit "disruptive behavior" also may be denied the right to act as their own lawyer, the prosecutors argued.

"This court is well aware of Mr. Hinckley's history in this regard," the prosecutors said.

The government lawyers also contend that Parker lacks the power to ease restrictions on Hinckley because such limitations are matters of internal hospital policy.