THE MOST distinctive ruin in Mexico is probably the pyramid-shaped structure called El Castillo, which dominates the site of Chichen-Itza in Yucatan.

Similarly, Yucatan can claim the most distinctive cuisine of Mexico. Well known dishes include cochinita pibil and pollo pibil, which are respectively, suckling pig and chicken coated with achiote paste, wrapped in banana leaves and cooked in pits in the ground.

Achiote, the orange-red seed of the annatto tree, is a typical Yucatan condiment. Mild in flavor, it adds brilliant color to a variety of dishes.

In Yucatan, achiote is ordinarily used in the form of a paste seasoned with other ingredients such as garlic, oregano, cloves, cumin, salt and pepper. w

The paste is sold in bulk in the big municipal market of Merida, the capital of the state of Yucatan. It also is packed in small boxes for export to other parts of Mexico and to the United States, where it can be found in markets dealing in Mexican ingredients, (among them the Latin Markets in Adams-Morgan).

One brand is labeled achiote condimentado and another adobo de achiote. The paste is also called recadeo colorado and recado rojo, the word recado referring to the mixture of ingredients and colorado and rojo to its red color.

The paste makes a colorful coating, as well as a delicate seasoning, for a pork roast to serve as a main dish of a Mexican dinner.

In Yucatan, the paste would be blended first with sour orange juice, then smeared over the meat. Since sour oranges are rarely, if ever, available the paste can be blended instead with mild vinegar, which also is used in Yucatan, or with orange juice. If there is time, let the meat stand to marinate in its achiote coating before baking.

Accompaniments include a dish of squash and corn, which is a popular combination in Mexico, and white rice cooked in chicken broth and flecked with parsley. Orange and onion slices and stuffed green olives are arranged over avocado halves for the salad.

The party starts with Peach Margaritas, developed in response to a reader request. Either fresh or canned peach slices are blended with gold tequila, lime juice, triple sec and ice. This brightly colored, tropical-tasting drink would be well suited to a brunch and ideal in hot weather.

For appetizers, serve quesadillas made by melting cheese atop flour tortillas, then spreading the tortillas with guacamole. Two large tortillas will make several servings. Cut them into wedges with kitchen scissors, which will do a neater job than a knife.

Dinner ends with a light and elegant Kahlua mousse, made with chocolate but less rich than the usual chocolate mousse. For a decorative effect, the mousse was garnished with a sparkling border of unflavored gelatin combined with water and sugar, chilled and cut into cubes.

A mellow white wine, rose or sangaria would go well with the pork along with bolillos or other dinner rolls. PEACH MARGARITAS (4 servings) 6 ounces (3/4 cup) gold tequila 3 ounces triple sec 2 tablespoons lime juice 4 teaspoons sugar 2 cups sliced fresh peaches or 2 cans (1 pound each) sliced peaches, drained 8 ice cubes

Combine tequila, triple sec, lime juice, sugar, peaches and ice cubes in blender. Blend until peaches are pureed. Serve over additional ice in salt or sugar-rimmed glasses. QUESADILLAS WITH GUACAMOLE (8 servings) Two (12-inch) flour tortillas Butter or margarine 1 cup shredded Monterey Jack cheese 1 cup shredded longhorn cheese 2 cups guacamole 2 tomatoes, cut in wedges Cilantro sprigs

Spread tortillas lightly with butter and place on baking sheets. Top each evenly with 1/2 cup Jack and 1/2 cup longhorn cheese. Place under broiler just until cheeses melt. To serve, spread each with 1 cup guacamole and garnish with tomato wedges and cilantro sprigs. Cut in wedges to serve. GUACAMOLE (Makes 2 cups) 2 avocados 1 tablespoon lemon juice 4 slices bacon, cooked and crumbled 3/4 teaspoon garlic salt 1/4 cup grated onion 2 tablespoons finely chopped green chiles Dash hot pepper sauce (optional)

Peel and mash avocados. Combine with lemon juice, bacon, garlic salt, onion, chiles and hot pepper sauce, mixing well. ROAST PORK YUCATAN STYLE (6 to 8 servings) 2 ounces achiote paste 3 tablespoons orange juice 5-pound boneless pork roast Salt, pepper Garlic powder

Blend achiote paste with orange juice until of spreading consistency, Coat pork roast with achiote mixture, then season to taste with salt, pepper and garlic powder. Place in roasting pan and bake at 325 degrees 2 1/2 hours. a SQASH AND CORN (6 servings) 2 tablespoons oil 1 small onion, thinly sliced 1 clove garlic, minced 1 pound zucchini, cut in 1/4-inch slices 1 can (12 ounces) Mexican corn 1 cup chopped tomatoes 1/4 to 1/2 teaspoon oregano Salt, pepper

Heat oil in skillet. Add onion and garlic and cook until onion is tender. Add zucchini, corn, tomatoes, oregano and salt and pepper to taste. Cover and simmer until squash is tender, about 15 minutes. WHITE RICE (6 servings) 1 1/2 cups rice 3 tablespoons oil 1 small onion, chopped 1 clove garlic, minced 3 cups chicken stock 1 tablespoon minced parsley

Soak rice in hot water 15 minutes. Rub rice until water is cloudy. Drain rice and rinse well with cold water. Drain thoroughly and spread out on a platter to dry.

Heat oil in Dutch oven or large, deep skillet. Add rice and cook until golden. Add onion and garlic and cook until onion is tender. Add chicken stock and parsley. Cover and boil until most of the liquid has been absorbed, then steam rice over low heat for 30 to 45 minutes, or until tender. AVACADO SALAD (6 servings) 3 tablespoons olive oil 3 tablespoons vinegar 1/4 teaspoon salt Dash pepper 3 medium avocados 2 tablespoons lemon juice Crisp salad greens 1 medium orange, peeled and sliced 1 small onion, thinly sliced 1/4 cup sliced pimiento-stuffed green olives

Blend oil, vinegar, salt and pepper and set aside. Cut avocados in half and peel. Brush cut surface of each hald with lemon juice. Arrange avocado halves on salad greens and top with orange and onion slices and olives. Sprinkle with dressing. KAHLUA MOUSSE (6 servings) 1 envelope unflavored gelatin 1/2 cup sugar 1/8 teaspoon salt 1 cup milk 1 package (6 ounces) semisweet chocolate pieces (1 cup) 1/4 cup Kahlua 1 teaspoon vanilla 1 cup whipping cream, whipped

Mix gelatin, sugar and salt in a 2 1/2-quart saucepan. Stir in milk and chocolate pieces. Place over medium heat and stir constantly unitl gelatin is dissolved and chocolate is melted. Remove from heat and beat with rotary beater until chocolate is blended. Stir in Kahlua and vanilla. Chill, stirring occasionally, until mixture moulds slightly when dropped from a spoon. Fold in whipped cream. Turn into a 4-cup mold and chill until firm. To serve, unmold and garnish with shaved chocolate or additional whipped cream, if desired.