Marjorie Merriweather Post, the Postum and Post Toasties heiress, was famous for her collections of international leaders, high society personages, mansions, yachts, airplanes, gems and decorative arts.

Now, almost 10 years after her death, a major part of the Post collection of Indian art and artifacts is open to the public for the first time. The objects were originally collected for Topridge, her immense Adirondack "camp" of 42 guest houses and dependencies, near Saranac Lake in New York State. The camp was left to the state.

She left the Indian artifacts to the Smithsonian, but only a few have been exhibited. Now a third of the collection, 190 of the best objects, has been put on long-term loan to the new museum at Hillwood.

The Indian exhibit is housed in a completely new structure just built on the 25-acre Hillwood estate in Forest Hills in the District. The new museum may well be counted the most charming, though the most modest, of any Marjorie Post building.

The Indian artifacts--rugs (to be changed periodically), quillwork, pottery, baskets and feather headdresses--are the sort most people will find fascinating--from small children who only know Saturday morning television Indians to the ever-increasing collectors of Indian art.

(The main mansion at Hillwood houses French and Russian decorative art, including Faberge' eggs and objects. A "dacha" holds more Russian art. As one Hillwood official put it, "The Indian exhibit is for beer-drinkers like me, who might not be as interested in the delicate stuff. It's intended to balance the exhibits.")

The 30-foot-high, one-room building is a wonderful 1980s interpretation--not a pastiche--of the indigenous Adirondack log cabin. The log structure gives a warm, woodsy, forest shelter feeling, without looking like a Wild West set. Sara O'Neil-Manion, project architect, designed the building to be rustic without being cutesy.

You come into the building through double doors from an expansive covered porch. Inside is a dark hall, which opens into the 30-foot-high, balconied space.

The glass inside door is framed by two 18-foot-high oak trees, branched at the top, almost 30 inches around. Another 11 oak trees, 12 feet high, cut carefully so their base retains the natural bell root base, support the great log band at the top of the wall. A huge stone fireplace, tall enough for a short person to stand in, rises the whole height of the north wall. Large windows take advantage of the acres of forest land in the middle of the city.

The roof is heavy timber construction using real beams. Bark-edged cedar siding covers the outside. Hemlock logs, almost 12 inches around, are sawn and stacked for the inside wall. The oak floor is pegged. O'Neil-Manion's only regret is that District regulations made a masonry firewall necessary in the middle of the sandwich. "Otherwise, the roof would really have been supported on the wood walls," she said.

The design owes much to the Arts and Crafts Movement, popular at the turn of the century, when wood craftsmanship was much admired. O'Neil-Manion and her partner, William Manion, the project manager, went to Topridge to study its main lodge, in the 1920s log cabin-mansion style. E.A. Baker, the contractor, is responsible for the remarkable woodmanship.

Antique photographs and helpful labels help explain the artifacts. The objects seem to have been chosen for their charm. Most are housed in handsome glass-doored cases set into the walls.

A large rug, woven in the 1920s by a famous Navaho shaman/weaver, Hosteen Klah, is remarkable because it uses the patterns of sand paintings. A headdress, perhaps made for a Wild West show or for a Shrine convention in Washington, is feathered and fierce.

The Acoma clay water jar, painted with stylized animals and curlicues, dates from the early years of the century.

A Yokuts coiled jar, made of grass, redbud, feathers and yarn is an intricately made piece. Bead pocket books with small-bell shaped bangles are made from throw-away finds.

Silver ashtrays and silver boxes, once scattered around Topridge, give a good idea of the way Marjorie Post used the artifacts as decorative objects.

Anthropologist Priscilla Rachun Linn was the director and curator of the show, installed by Patricia Chester of Root and Chester Design Inc.

The Indian exhibit and the gardens are open from 11 a.m. to 4 p.m. daily except Tuesdays and Sundays, but visitors must call 686-5807 for reservations on the day they wish to attend. The French and Russian decorative art in the mansion is open only by advance reservation at the same number.