The Decorator's Show House, as always, benefits the National Symphony Orchestra under the sponsorship of the Women's Committee, chairmanned by Marcelle Cahill. The house, after an invitational preview last night, is open tomorrow through Oct. 22, from 10 a.m. to 3:30 p.m., weekdays and from 12:30 p.m. to 6:30 p.m., Sunday. Admission is $5. Babies or children under 8 years are not welcome.

All the decorations and furnishings will be for sale after the show house closes. An antique shop, boutique and tea room are also offered. Parking this year will be simple -- on the grounds. Call 785-8100 for information.

OTHER SHOW HOUSES:

In Wilmington, Dela. -- The Wilmington house, through Oct. 14 sponsored by the Wilmington Opera Society Guild, is at Nemours of Montchanin, on Route 52. There is shuttle service from Philadelphia airport. (Call 302 656-3992 for directions.) The 1824 house is one of the older duPont homes, built for Alfred Victor duPont. The present owner is J. Simpson Dean, Jr., a great-great-grandson of the original owner. Twenty-two decorators have worked on the house. The house is open weekdays 10 a.m. to 4 p.m., Sundays 1 to 5 p.m. and Wednesday nights until 8 p.m. Tickets are $4.

In Charlottesville, Va. -- The Univsersity of Virginia's Birdwood house, is the designer house sponsored by the University League of Charlottesville-Albemarle to benefit the University Hospital's health programs. The 150-year-old house on a 550-acre estate has been a faculty residence. Washington designer Suzanne Shaw has done a room with a new fabric called Birdwood, from the new Charlottesville line by Waverly Fabrics. The house is open weekdays 10 a.m. to 4 p.m. and Sundays 1 to 5 p.m., Wednesday and Friday evenings from 7 to 9. Tickets are $4. For information call Sharon Watlington, 804 977-4910. The house is a mile and a half west of the university on U.S. 250.

In Annapolis, Md. -- The Annapolis weekend centers around the town's oriental influence that sailed in during the 18th centruy.Porcelain from China was brought in by ship captains. in 1852 Admiral Perry on the U.S.S. Mississippi sailed from Annapolis to reopen Japan for foreign trade. Oriental taste influenced the dome on the State House, the Chinese Chippendale staircase and the garden of Paca House with its Chippendale tracery bridge and pond, copied from Nanking china.

On Sunday, exhibits will be of oriental porcelain, Japanese prints, bonsai flower arrangements, 1870-1930 photographs of the orient, and oriental treasures brought back from Naval officers. Sunday events include open house at five buildings newly restored by Historic Annapolis. Price for the 11 a.m. to 5 p.m. events is $8. For information call Historic Annapolis, 301 267-7619.