Pulling up to the red barn in Patuxent River Park, a visitor wonders if this is really a park. There are two other cars in the lot. On a warm Sunday afternoon, the only people in sight are two naturalists and a family of four.

This is what Maryland-National Capital Park and Planning Commission calls a "limited-access" park, where visitors must apply for a permit to use it - to hike, fish, boat, canoe or ride horses. It's one way to keep a natural area natural.

From the park, a wooden tower overlooks the Patuxent and Jug Bay. Off in the distance, a great blue heron wades in the tidal marshes. There's Mount Calvert, nothing more than a rise, once the site of the original Prince George's County courthouse. A line of trees marks where an old railroad line took tourists to the Chesapeake and back, on a train that smelled like fish.

Park director Richard Dolesh isn't sure why it's called Jug Bay. Maybe someone thought the shoreline looked like a jug, or maybe the marsh was once filled with "a bunch of drunkards and stills. Oldtimers talk about the stills that used to be in the marsh," said Dolesh. "They said if you ever found one, you just kept going."

Nowadays the way through some of the marsh is 3,000 feet of boardwalk. Jewelweed, water hemlock, wild rice and marsh mallow are all within stooping distance, crowding against the walk in lush wildness. There's skunk cabbage that when crushed smells worse than skunk, and arrowwood, which Indians used to make arrows.

Dolesh pointed out the dragonflies and called them "snake doctors, because they're always together." With snakes. Oh. It does help to have a naturalist along. "Most people who haven't grown up in an outdoor setting feel threatened," said Dolesh. "They feel they don't know enough." True.

Close to the river, a photography blind gives visitors a quiet place to sit and watch. Osprey live on a platform nearby. When they hatched, Dolesh says, he felt downright paternal.

Canada geese and swans winter over here. Downriver is the Merkle Wildlife Management Area, once owned by Edgar Merkle. He had a flock of tame geese, says Dolesh, and their descendants keep coming back and bringing wild birds with them - now as many as 20,000.

The park's small staff guides group tours on electric-powered boats through the marsh. They also do interpretive progams on life a hundred years ago in southern Maryland. Among the exhibits is a 150-year-old chestnut log cabin rescued from the woods and reassembled, and a tobacco packing house that shows how the chief local crop was handled. Reservations are needed several weeks in advance for interpretive programs throught the marsh and the village mock-up.

Permits and reservations are a nuisance. But then, so are crowds. "People who are up to no good and want to throw beer cans in the river," says Dolesh, "they aren't so anxious to fill in the application."

The park's role, as he sees it, is to help develop an appreciation for the river - "a sense of responsiblilty for it." And when groups phone in to use the park, he coordinates their visits, "so we don't have the Maryland Ornithological Society at the same time as 300 screaming kids on a camporee."

COMING UP ON THE RIVER

True, it's not the sort of place you can just drop into- usually. But Saturday August 4, the rules will be relaxed for the Jug Bay raft race. On your raft, you can even drop into the river. Anyone can enter a home-made raft, as long as it floats, is safe and can be launched from a boat ramp. Registration deadline is August 1. A limited number of life jackets and canoe paddles will be available. On race day, rafts should be brought to the visitors' center and logged in before 1:30. Launch is at 2. Spectators and self-made rafters are free. (Call 627-6074 or 627-6075.)

WHERE TO FIND IT - Take Pennsylvania Avenue way out to where it becomes Route 4 and take it to U.S. 301 south. Turn left on Croom Station Road. Turn left on Croom Road and continue until you get to Croom Airport Road. Turn left and follow Croom Airport Road to the park.

THOSE PERMITS - Call ahead to have a permit application mailed to you. Annual parking permits, available to residents of Prince George's and Montgomery counties, cost $3. Annual parking and boating permit, $12. Non-residents pay a day-use rate of $3 to park and $3 to use the boat ramp. Canoes cost $6 a day to rent and must be reserved in advance. Call 627-6074. CAPTION: Picture 1, A LITTLE HOUSE FINDS A NEW HOME IN PATUXENT VILLAGE.; Picture 2, LAUNCHING FROM A DOCK OF JUG BAY. Photos by Pamela Whitehead.