The trial of a District man, accused of literally frightening to death a motel clerk during a robbery last year in Chevy Chase, began yesterday in Montgomery County Circuit Court.

Michael Stewart, 20, pleaded not guilty by reason of insanity to the charges of felony murder, robbery and attempted robbery.

The case involves the death of Pearl A. Pizzamiglio, 60, a clerk at the Chevy Chase In Town Motor Hotel at 6800 Wisconsin Ave. Prosecutor Richard Jordan told the jury that Pizzamiglio was so frightened by four young robbers on Nov. 19, 1983, that she suffered a fatal heart attack caused by stress after the men stole $167 from her in the motel lobby.

Under Maryland law, felony murder is committed when someone is killed as the result of a felony such as rape, robbery or kidnapping and also includes any accomplices as being responsible for the death. Although Pizzamiglio was not physically attacked by the men in the robbery, felony murder convictions have been won in similar cases involving stress-related heart attacks in Arizona, California and New Jersey, according to a spokesman for the state's attorney's office.

Three other men charged with murder in the case have pleaded not guilty and will be tried separately.

Stewart's attorney, Barry Helfand, argued that his client, who he said was subdued with antipsychotic medicines during his appearnce yesterday, was mentally incomptent the night of the robbery because he frequently used the street drug PCP.

Helfand told the jury they must decide, "Is he guilty but insane? That doesn't mean he's going prancing out the door home. You'll hear about the mental hospital he'll go to."

The first witness, Montgomery policy officer Nancy Calder, said that she responded to Pizzamiglio's call for help and found the clerk "extremely scared . . . pale in appearance . . . nervous and jittery."

On the way to police headquarters, Pizzamiglio complained of chest pains, according to police records in the court documents. At 1:28 a.m. Nov. 20, she was pronounced dead of a heart attack at Suburban Hospital, Jordan said.