A 13-year-old girl suffering from cerebral palsy drowned yesterday when the wheelchair to which she was strapped rolled into the waters of Baltimore's Inner Harbor.

The victim, identified as Dianca Jennings, lived at the Bello Machre home for the mentally retarded in Glen Burnie, Md. She was among a busload of about a dozen youngsters from the home, supervised by three adults, who had just disembarked from a boat tour of the harbor.

Police said one of the mentally retarded boys had been pushing her wheelchair and apparently lost his grip. The wheelchair rolled down an incline in front of the Pratt Street pavilion of Harborplace and plunged from the brick plaza, which has no guardrail, into the water near the berth of the USS Constellation. The drowning occurred at 1:35 p.m.

A bystander told WBAL radio that he and another man tried to save the girl. Police divers pulled the wheelchair and the girl's body to the surface shortly after that attempt. An autopsy will be performed and the investigation is continuing, police said.

Dr. Robert Ireland, executive director of the home, said everyone connected with the facility is "just overwhelmed by the whole tragedy." It was his understanding, he said, that the girl's wheelchair was on the dock and stationary just before the accident. It was not clear what happened then, he said.

An employe said that at the cottage where Dianca Jennings lived supervision is around the clock. However, the employe said, some staff members had expressed concern over what they viewed as possible inadequate supervision of residents on field trips.

Ireland responded last night that he "can't believe any staff person would say that." He said there were "10 or 11" clients supervised by three staff persons on the trip, a supervision ratio that he called "excellent by anyone's standards."

Baltimore's Inner Harbor is a favorite tourist attraction since its recent redevelopment as the site of the National Aquarium, the Maryland Science Center, the World Trade Center, Civic Center and Harborplace, a complex of fashionable shops and restaurants.