A day-old baby boy was kidnaped last night from his room in George Washington University hospital when his mother left the room briefly in response to a telephone call, authorities reported.

Police were searching the hospital and grounds late last night, but as of early this morning the child had not been found.

According to a hospital spokeswoman, the mother, identified as Linda Jackson, 24, received a telephone call about 8 p.m. in the third floor maternity ward of the hospital.

A woman's voice asked the mother if the baby was in the room. When Jackson said he was, the caller asked the mother to go down the halfway to sign papers, presumably in connection with the boy's birth, the sporeswoman said.

The mother found no one awaiting her and returned to her room in about five minutes. When she did, authorities said, the child was gone.

Authorities said they had not indication of who might have taken the child from his crib or of any motive in the kidnaping.

They said the mother and child were the only occupants of a semiprivate room in the newborn wing. About 24 other newborn babies were in the section last night.

The child was described by a physician at the hospital as healthy and weighing about 5 pounds, 11 ounces. The doctor said the child would require "fairly ordinary newborn care," including regular feeding and stable temperature.

The mother was identified by the hospital spokeswoman as a resident of Southeast Washington. A police detective said he underdtood the mother lived in the Hillside section of Prince George's County. No further details about her were immediately available.

The spokewoman said that it could not be determined who had made the call to the mother's hospital room or whether the call had come from within the hospital or outside.