THE BRANDYWINE River winds its leisurely way through some of the loveliest country in this part of the world. En route to the Delaware River, it meanders through southeast Pennsylvania, its banks overhung with Victorian farmhouses built on land known largely to pheasants and spavined horses. Overlooking the Brandywine, George Washington once encamped with too small a force, awaiting the attack of Gen. Howe. Some 125 years later, Howard Pyle, the famous illustrator, pedaled his bicycle along its banks from Wilmington to Chadds Ford and decided that nowhere else on earth could his art students find more quiet beauty for inspiration.

At Chadds Ford, the little road bends and spills you out into a clearing where the Brandywind River Museum sits at a turn of the river. This is the famous home of many Wyeth paintings. But even if you didn't go inside to look at them, you would be repaid for the trip. The museum itself is a work of art that seems as much a part of the countryside as the Brandywine itself.

The building is an ancient grist mill saved from the bulldozer by the Tri-County Conservancy of the Brandywine. There could hardly be a more attractive setting for the art of three generations of Wyeths and of Howard Pyle and the artists of the region who studied at Chadds Ford. One enters through a courtyard laid with Belgian block from the streets of Philadephia, studded with the inset patterns of old millstones. Rough white plaster walls and the original hand-hewn beams are paired with a huge glass column that overlooks the river to let nature compete with the art of the galleries.

The people of Chadds Ford hang on the white walls in the Wyeth paintings. Karl Kerurner, Wyeth's neighbor, is the snow-encased figure in "Spring," the gallery's new addition. Jimmy Lynch, who is always somewhere in the museum, posed for Jamie Wyeth's "Draft Age." The buttonwood trees of N. C. Wyeth's Scribner illustrations of "Robin Hood" are the trees of Chadds Ford, the Admiral Benbow Inn in "Treasure Island" his own house. Howard Pyle's drawings and those of many of his students, including Maxfield Parrish, hang on walls near where they were painted.

Everywhere through the glass the river beckons. You can walk out the museum door and onto the nature trail the Conservancy maintains, or sit on a gristwheel and watch the water eddy by. You can buy sandwiches in the museum cafeteria and picnic at the tables on the river banks, or rent a canoe upstream at Lenape and float past.

A whole day is not too much at the Brandywine River Museum, but across Kennett Pike beckons the manicured horticultural splendor of Longwood Gardens, another treasure of the valley. It is Pierre Samuel du Pont's gift to the public, and 600,000 of us pour through its gates every year. This year a new vistor's gallery has been added, and a visit is a must.

The fall season has just been inaugurated at Longwood, and a 4-minute film presentation as you enter will help you decide which of the 300 acres will entice the most. A helpful model of the estate lights up in the appropriate section when the film touches on its attractions.

Longwood is studded with trees that are nearly 200 years old, ginko, cucumber magnolia and bald cypress that tower above the walkers and link the original owners of the land to the present. Elms threatened with extinction elsewhere flourish here in a beautiful allee, vaccinated against the dread Dutch elm disease. The trees are the reason Longwood exists today. On a Sunday drive in 1906, du Pont observed that a number of beautiful old elms had been marked for cutting and bought the land to save them.

Fall at Longwood is a blaze of chrysanthemums, but they are not hardy and will die with the first frost. No matter. The gardens themselves, the designs, the fountains are breathtakingly handsome with or without flowers. Especially impressive is the open-air amphitheater with its wings of clipped, arbor vitae and its curtain of leaping fountains, which screen the stage between acts. Italian and Oriental statuary, topiary gardens and reflecting pools are here the year round. Longwood has no closed season.

Four acres are under glass at the conservatory of Longwood. Columns covered with creeping fig recreate the trees outside, and the sun pours in from the glass roof on emerald grass.In November, 15,000 chrysanthemums will be cascading down the pillars, and in December the place will be a sea of poinsettias. For those who wander gardens in search of ideas to bring home, a special exhibit of suggestions for city balconies will be displayed through Christmas.

Tired? Sit on one of the stone benches and watch the fountains play. You can't flag now. Winterthur is just down the road. And I hope you phoned ahead for the special tour that shows you more than the usual standard rooms in the Washington wing. (The number is 302 -- 656-8591; ask for Jan Clark.)

New this year at Winterthur is a one -- hour tour by trackless trolley of the famous gardens, which had their beginning when Henry Francis du Pont, just past 20, planted a few bulbs on the slope by the house. Over the years the gardens have become a blending of woods, brook and ponds suggesting an English park. The fall foliage is brilliant. Canada geese, swans and a pair of unusually curious turkey buzzards make their home here, sharing it with the visitors who walk the paths every year. One of the nicest ways to enjoy all this is to walk around a bit and then lunch in the garden pavilion, where you can enjoy the vista along with a tasty meal.

As nearly everyone knows by now, du Pont began Winterthur by collecting the interiors of old houses built between 1640 and 1840 along the Eastern Seaboard, installing them in the house that had been built by his great uncle in 1839. He filled the rooms with American furniture of all periods before the Industrial Revolution, and you can wander through the centuries at Winterthur, room by room.

Only the Washington wing is open without advance notice, an 18-room sampler of how our ancestors lived, from a New England tavern room to a Charleston dinner party. With advance you can add several rooms, including the one containing Chinese exports, where you should be sure to see the Ming dynasty plate saved from the 1613 wreck of the Dutch ship, the White Lion. It was dredged up last year undamaged, having been cushioned by a bed of peppercorns.

You'll also get to see Shop Lane. Wander through this re-created back street, which ends in the End Shop, where they will show you the pornographic machine featuring a gentleman who lifts his fig leaf when emerging from his box. The special tour also takes you through du Pont's own high-style Chippendale living room, furnished in the taste of the '30s, so symmetrical that each part of the room is a mirror image of the other.

It's all very dazzling, but one's eye does begin to glaze eventually and one's feet demand a recess. The Chadds Ford Inn at the corner of Rtes. 1 and 100 is a good place for a restorative meal.

The tavern, dating back to 1736, is a part of the history of Chadds Ford and must have been in the line of fire when the American forces retreatd up the valley to Chester before Howe's army. It once served travelers fording the Brandywine on their way to the growing city of Philadelphia, and its front windows have overlooked a good bit of the history of the country passing by.Today it is a tavern wrapped in the patina of age, a small restaurant and bar (no rooms) specializing in fish and veal.

For overnight stays you must look elsewhere. The Mendenhall Inn on the Kennett Pike is attractive, though you have your choice of motels as well. Mendenhall has just 15 rooms, all nearly always occupied, so plan ahead if you want to enjoy its walled patios, pretty little lobby featuring a free-standing fireplace, brick floors and an atmosphere that fits the surrounding countryside. Mendenhall also serves meals.

If you have any energy left, the Delaware Museum of Natural History across from Winterthur is a nice place to check on the birds and animals of the area. Just beyond Chadds Ford Inn is the Brandywine Battlefield Park, where the farmhouse in which Washington stayed in 1777 has been restored and put on view.

Just pack your walking shoes and start early. The Brandywine Valley is crammed with delights.

Getting There: To see the sights of the Brandywine valley, take I95 to Wilmington. Pick up Delaware Avenue, which becomes Kennett Pike (Rte. 52) north and go through Greenville and beyond about a half mile to Kirk Road and turn right. In another half mile this hits Rte. 100. Turn left. (Because of construction, you cannot take Rte. 100 all the way from Wilmington.)

The Brandywine River Museum is on the left just before Rte. 100 intersects with Rte. 1 in Pennsylvania. Chadds Ford Inn is on Rte. 1 northeast of the museum, and Brandywine Battelfield lies just beyond. Longwood Gardens are west of Rte. 52 off Rte. 1 and Winterthur can be reached by doubling back on Rte. 52. Mendenhall Inn is on Kennett Pike between Winterthur and Rte. 1.