Some 9,000 restaurateurs were in town recently for a regional restaurant exposition at the Sheraton Washington, hosted by the Restaurant Association of Metropolitan Washington.

Many of the visitors took a couple hours out of their schedule to drive out to 1300 Rockville Pike, the site of a new eating establishment called Mother's Pizza Parlour & Spaghetti House, a theme restaurant with moderate prices that caters to families.

The Rockville restaurant opened three weeks ago, the first of what is planned to be a Maryland-Virginia-D.C. chain of Mother's, a highly successful business concept imported from Canada. The restaurant's interior recalls the 1925-35 era with loads of antiques and collectibles.

Much of the delight of Rockville Mayor William Hanna Jr., Mother's also features a "Rockville Room," dedicated to the city's history as told through local bric-a-brac, signs and photographs.

With 200 seats, the $1.5 million Mother's in Rockville will employ 120 persons and feature medium-priced meals, a large salad bar and desserts. Mother's opened first in 1970 in Hamilton, Ontario, and has grown to about 50 restaurants with annual sales projected at $60 million in 1980. All the restaurants are similar in concept but each has its own character.

In the case of Rockville, Mother's signed a licensing arrangement with 34-year-old Jacob Middel, who toured the United States in 1976 and 1977, looking for a market in which he wanted to live and establish a restaurant business. Middel, a native of The Netherlands, was a partner in six of the Mother's restaurants in Canada.