Atmosphere: Informal.

Price Range: From $6.95 for family-style dinners to $10 for some specially ordered dinners. Children pay less, based on how much they eat.

Credit Cards: None accepted.

Personal checks are taken.

Hours: 5 p.m. to 10 p.m. Tuesday through Saturday.

Special Facilities: Parking in private lot behind restaurant.

Limited access to patrons in wheelchairs because of steps at entrance and inside house.

Reservations: Needed for groups of six or more.

We have a Mrs. Morton to thank for a delicious dinner out the other night. Mrs. Morton is the cook at our children's school, and when tacos were ordered for the menu, she prepared them with such panache that our daughter, began demanding Mexican food from other sources. She not only wanted me to experiment with tacos, refried beans and enchiladas, she began looking for restaurants that served such items. When she spotted Rio Grande's small sign on Rockville Pike, she began lobbying in earnest.

Fortunately, we had heard the Rio Grande was a good, inexpensive place to have dinner, with or without children in tow, and that if you don't go at prime time, you don't have to wait long in line.

We decided to try the Rio Grande on a Saturday night at 6:30. When we called, we were told that reservations are necessary only for parties of six or more and that if we arrived before 7, we would only have to wait 15 to 20 minutes for a table.

We followed another family of four into the Rio Grande, which occupies an old house on Rockville Pike. They got the last table. We were told the wait would be a few minutes and that gave us time to look around. Tables were set up in what must have been the living room, enclosed porch and dining room of the old house. A piano graced the living room while a wall of hanging plants was visible on the porch. There was also a waiting room lined with benches and blow-ups of past newspaper reviews. The waiters wore embroidered Mexican-style shirts.

As we were seated, our waiter put tostaditas compuestas (Mexican salads) in front of us and explained the Rio Grande system: The food is served family style, with cheese enchiladas and rice, tamales and beans, and dessert to follow. Adults pay $6.95 for the full course dinner; children pay less, depending on how much they eat. Seconds are offered in all hot courses.

Tostaditas compuestas turned out to be shredded lettuce and grated cheese on taco chips. Our waiter warned us that the accompanying sauce was very hot. Only the braver palettes at our table tried it. Our daughter liked it in small doses; my husband was grateful for the Mexican beer.

The cheese enchiladas and rice, still hot in their casseroles, were passed from table to table by a waitress and waiter. While we were served our first helping, other tables were being offered seconds. The cheese enchiladas were our familily's favorite course: smooth and nicely seasoned. The chicken tamales and beans appeared next, served in the same style as the enchiladas. We all liked the beans but none of us liked the tamales as much as the other dishes. Both courses were offered again before the dessert appeared.

The waiter called the last course bunuelas. It was a strip of nicely browned pastry with a slab of guava jelly on the side and a pitcher of honey for further sweetening. The reviews were mixed: My son and I loved it; my daughter and husband liked it better without the guava jelly.

The waiter told us the same dinner is served every night, but that special order dinners are available on request. These cost up to $10 per person.

Our daughter said she was even more enthusiastic about Mexican food now that she had had a real Mexican dinner. We enjoyed it and felt the dinner had been a good value. The four course meal plus two beers came to $23.10 - the children were half price, evidently not eating enough by Rio Grande standards to warrant adult fare. By their standards and ours, the children ate a fairly large meal. They finished everything they were served, but didn't take seconds.

Rio Grande, which serves beer and wine, is not the kind of place to sit around and relax over cocktails before dinner or over coffee after dinner. The efficient system of service precludes that. Besides, by the time we finished our bunuelas, the line of people waiting for a table at Rio Grande stretched out the door and filled the waiting foyer. Evidently, many guests came prepared - some were playing cards while they waited.