A 5-year-old Wheaton boy was bitten by a rabid raccoon Saturday, becoming Maryland's first known human victim of a rabies outbreak that has become increasingly serious in the Washington area.

Montgomery County health officials said the boy, whose name is being withheld, was playing on the C&O Canal towpath in the Great Falls National Park, a popular area for picnickers and hikers, when he was attacked by the raccoon and bitten on the right knee.

The boy's father was also attacked but not bitten, according to a county official.

The boy's father then killed the raccoon with a stick.

The raccoon was diagnosed as rabid on Sunday, when its brain was taken to a laboratory in Baltimore. The boy was taken to Holy Cross Hospital in Silver Spring late Sunday for the first of five injections of rabies vaccine.

County Health Officer Lewis Holder said three more persons who had contact with the dead raccoon were ordered to undergo the rabies treatment.

They were identified as two Boy Scout leaders who picked up the animal by the tail, and a park ranger who took it by the tail and put it in a bag.

At the same time, Holder said the county's free rabies vaccination program for pets has so far been a major disappointment. On Sunday, the first day of the program, only 170 dogs showed up. County officials estimated that there are about 20,000 unvaccinated dogs in the county. The next free rabies vaccinations will be given next Sunday 1:30 p.m. to 3:30 p.m. in the open space in front of the new County Office Building at 101 Monroe St. in Rockville.

Holder said health officials still believe the rabies outbreak is more dangerous to dogs and cats than to humans, since the pets tend to chase wild animals. The boy approached the raccoon after other children had been feeding it, said Edmond F. Rovner, special assistant to County Executive Charles W. Gilchrist.

County officials repeated their warning yesterday that people stay away from any wild animal. They said raccoons, when spotted in the daylight and appearing in a somewhat sluggish, inebriated state, are particularly suspect.

At least six rabid raccoons have been found in the county since mid-summer. The discovery of the first three prompted the free vaccination program. A fourth was killed last week on an island in the Potomac River by a Virginia hunter using a bow and arrow. The hunter removed his bloody arrow, and was forced to undergo rabies shots in Virginia.

Rovner said another possibly rabid raccoon was found yesterday morning.

The county has stepped up its efforts to trap raccoons, but Holder cautioned yesterday that there is no way to eradicate the county's raccoon population.