''YOU have exported your fast-food chains to our country,'' said Alfonso Ornelas, personnel manager for a chain of Mexican restaurants called Chon y Chano. ''Now we have given Chicago one of Mexico City's fast-food taquerias with our own particular style.''

The patrons of the first Chon y Chano in the United States, at 3901 West 26th Street here, in the largely Hispanic Little Village neighborhood, would argue that Americans got far better than we gave. The new restaurant is offering some of the best Mexico City-style antojitos (appetizers whose name means ''little whims'') north of the Rio Grande. If these soups and tacos are fast foods, they are traveling on a different track from American fast foods.

Mr. Ornelas and Adan De La O, one of the chain's owners, are justifiably proud of opening the first of the popular restaurants outside the Mexico City area, where there are five branches. This is the kind of brightly decorated urban establishment long popular with affluent Mexicans and knowledgeable foreigners, who stop in for a quick lunch or a late-night snack after an evening at a nightclub or theater.

''We still serve more taco varieties in our Mexico City restaurants,'' Mr. Ornelas said, ''but we get pretty close here, with 22 grilled meat and vegetable fillings. We are bringing in a chef from our biggest branch, and he will expand our menu even more.''

The Chon y Chano menu is impressive. For example, sopa Azteca, a central Mexican variation on a chicken and tortilla soup common across Mexico, is a chicken-tomato broth laced with fried tortilla strips, cheese, chili and lime. It is served with a large dried red chili pod, roasted until it puffs and crumbled into the soup for added flavor, texture and heat.

In the open tiled kitchen, the chef -two on busy weekends - also grills or griddle-cooks and chops to order taco fillings of boneless beef short ribs and steak, jerked and marinated pork, chorizo (spicy sausage) and brochettes of beef and vegetables. Bulbous green onions are grilled to a smoky tenderness and served with lime wedges for a traditional accompaniment. The fillings may be served with melted white Chihuahua cheese.

No-que-no combines grilled nopales (cactus flesh), bacon, mushrooms, roasted green chilis and cheese. Another rare treat for Americans is tuetanos al carbon (grilled marrow). Beef marrow, popular in Mexican appetizers, is ladled into a small pottery bowl and placed in the flames until it becomes a sizzling rich taco spread. At this rate, Chon y Chano's little whims could become serious food in Chicago.

photo of Chon y Chano in Chicago (NYT/Steve Kagan)