A 19-year-old receptionist for the Secret Service shot herself to death in the Old Executive Office Building next to the White House shortly after midnight yesterday, using a revolver she had removed from a locked office safe.

The body of Kimberley Acquard, of Alexandria, was found on the flloor in her small, first-floor office by a uniformed Secret Service agent on routine patrol about 1 a.m., District of Columbia police said. City medical examiner James C. Luke ruled the death a suicide.

A 38-caliber Smith and Wesson revolver, the type of gun carried by Secret Service agents who guard the president, was found nearby, sources said. Luke said the woman died of a single gunshot wound in the head.

Police said the woman apparently was upset because her boyfriend had told her he wanted to end their relationship.

According to police and the Secret Service, Acquard had worked in the Administrative Offices of the Presidential Protection Division for three years.

Responsibilities of the office include accepting weapons from agents who want them checked. Other weapons are issued to agents from a store of revolvers kept in the office.

The guns are kept in a locked office safe for which Acquard had the combination, officials said.

Acquard left work at 5 p.m. Wednesday as usual, police said, but returned to her office about midnight. Security guards apparently did not hear the single shot that killed her because it was muffled by the old heavy wooden doors that are used in the ornate 19th century building.