Richard M. Fields, 35, was found innocent by reason of insanity yesterday in the January, 1976, murder of St. Elizabeths Hospital attendant whom Fields shot while he was a patient there.

The government had claimed that Fields, who was accused of but not prosecuted for another murder at a Springfield, Mo., federal prison hospital 10 years ago, was sane at the time of the St. Elizabeths Hospital shooting and should be found responsible for his acts.

Fields' attorneys claimed that he was insane at the time of the St. Elizabeths murder, a fact they said was emphasized by his incarceration in a mental hospital at the time the murder was committed.

By accepting the defense claims and finding Fields insane on the murder charge, the jury set the stage for Fields' release on the murder charge when he is deemed sane. Ironically, by claiming during the trial that Fields was sane, the government now is in a difficult position to say he is insane and unfit for release at a future hearing on that issue.

If Fields had been found guilty, he could have been sentenced to life imprisonment.

Fields had returned from a pass on the St. Elizabeths grounds Jan. 23, 1976, when he shot and killed hospital attendant John F. Marr. In the last two years, various legal proceedings have been held about Fields competency to stand trial on the charges, with psychiatrists often disagreeing widely on Fields' sanity at various times.