An on-duty Montgomery County police officer has been charged with raping a 15-year-old Bethesda girl early Sunday morning while the girl and a friend were attemtping to aid police in an investigation, Montgomery County police reported yesterday.

After the alleged rape the officer, Joseph E. Baltimore, 33, of Rockville, a six-year veteran of the force, drove the girl and her friend back to the friend's house, where the two teen-age girls called other police, according to police spokesman Cpl. Phillip Caswell.

Baltimore was ordered to return to his base immediately and charged with the rape offense at the Bethesda station, to which he was assigned, and held in lieu of a $5,000 bond. He was released on Sunday and immediately suspended for 72 hours pending a further police investigation, Caswell said. A preliminary hearing on the matter has been set for March 23 in Montgomery County District Court, He added.

Baltimore could not be reached for comment. The name of the victim and her friend were not released.

According to Caswell, Baltimore, a uniformed officier driving a marked police cruiser, met the two girls at St. Dunstan's Church, on Massachusetts Avenue, at about 3 a.m. Sunday morning, and drove them in his police car to the 4500 of Sangamore Road, near the Brookmont Elementary School.

Caswell said Baltimore "took the girls to the location in reference in another police investigation," which police would not discuss.

Caswell declined to say whether Baltimore had been formally assigned to that unidentified investigation.

At the location, Baltimore and the 15-year-old girl got out of the car, while the other girl stayed in the police cruiser. The rape allegedly occurred some distance away, according to Caswell.

The girl was later examined at Suburban Hospital, though police refused to reveal the results of the examination, according to Caswell.