A week-old baby girl was found yesterday afternoon in a cardboard carton on a sidewalk in the Chevy Chase section of upper Northwest Washington.

The infant, who was found about 5 p.m. in the 3200 block of McKinley Street, was taken to Children's Hospital, where she was said to be in good condition. She later was released to D.C. government custody.

Police said authorities would try to find the baby's mother, but said they had no idea who abandoned the child.

Ellen Siegler, who lives in the neighborhood, said she had just arrived home yesterday when a man approached, asked if she lived there, and then told her that "somebody left a baby here."

Siegler said she went to the carton, looked inside and found the infant, her head wrapped in what appeared to be a bedsheet. "I was very surprised," Siegler said. "I can't imagine she'd been there very long."

After unwrapping the child's head, Siegler took the infant to her own nearby house and telephoned police.

The infant cried a bit, but "not hysterically," Siegler said. She "seemed pretty alert and moved a little . . . . She seemed to be okay."

A parent herself, Siegler said she tried to clean the infant, and joined a neighbor in diapering the baby, who had not been wearing one.

She said she was getting bottles to feed the infant when police arrived.

A small crowd gathered on the quiet street after the discovery, but Siegler said none of her neighbors saw who left the baby or the carton.

A supervisor in the city's Department of Human Services said last night that the child was in the agency's care.

Authorities said standard procedures call for a court hearing at which the baby could be placed in the city's custody.