A 35-year-old Washington man straddled a second-story windowsill for nearly five hours early yesterday, cradling his 9-month-old niece in his arms and threatening to drop her to the ground if police approached him.

The predawn vigil finally ended when the infant was pulled from her uncle's loosening grasp by an officer while a fellow policeman rushed the man from inside the apartment at 3184 15th PI.SE.

The man was identified as Gerald Thomas, who police said, is a former patient at St. Elizabeths Hospital and a onetime drug addict. He was examined later at St. Elizabeths, released and was charged by police with threat to do bodily harm. He was being held last in the central cellblock in lieu of $500 bond, police reported.

The child, Trayonne Nicole Dorsey, suffered only a bump on the head when she struck the roof of a porch and was pulled from Thomas' grasp by seventh district officer Freddie Adams, who had climbed to the top of the porch.

Trayonne was treated at Greater Southest Community Hospital and released after she was found to be in good condition, hospital officials said.

The diaper-clad baby withstood the ordeal better than the other, adult members of her family. During her long stay on the windowsill, she remained awake and tearless.

The drama began about 2:30 a.m. after Thomas had argued with members of his family, punched his mother and grabbed Trayonne. Authorities said the incident apparently was triggered by Thomas' contention that his family was conspiring to kill him.

A D.C. government psychiatrist arrived on the scene about 6:15 a.m. - about an hour before Thomas was to loosen his hold on the infant as he was rushed by a policeman - and told officers that Thomas apparently was beginning to "hallucinate."

It was this conclusion, authorities said, that prompted them to decide to move on Thomas.

As Officer Adams clambered to the roof of the porch, another policeman rushed Thomas from inside the apartment. Thomas began to lose his grip on the baby, and Adam made his successful grab for the child.

At this point, Thomas, clad in dangarees and a striped shirt, Lunged for Trayonne as he was grabbed from behind. Thomas- movement broke him from the officer's grasp and he tumbled from the sill and into a bush below. He was not hurt by the fall, police said.

The baby's mother, Gladys, witnessed the early part of the drama, but became upset and was escorted to a police cruiser, authorities said. She returned to the scene after her child had been rescued.

Thomas was taken from the scene to St. Elizabeths. He was released about five hours later as "not in need of psychiatric care," and was promptly arrested and charged by police.

A St. Elizabeths' spokesman said Thomas had been a patient there on three or four occasions since 1976, but declined to discuss details of his care because of patient cenfidentiality.

During the 5-hour vigil at the complex, which includes 12 apartments with entrances facing a common, grassy area, police said Thomas complained that his mother, his sister and another person were conspiring to kill him.

Officer Frank Roberson, one of the policemen inside the apartment, recalled after the incident:

"I"ll tell you what fascinated me about the whole thing. During the whole ordeal, the baby didn't cry. Once I looked up and there she was, just a cute little thing, playing with his (Thomas') beard."