The distinction between a mediocre and a good restaurant is a subtle one that the new management of Las Pampas has apparently understood. Thus Washington's only Argentine restaurant now has more claim to attention than its uniqueness.

Though Las Pampas is primarily a grill, emphasizing baby beef cooked over coals in its window as well as on its menu, some of its greatest assets are on the appetizer list. Five cold appetizers, five hot appetizers and three soups are not an impressive number, but their variety is special. The empanadas, for instance, two deep-fried turnovers with prettily braided edges, are spectacularly good ones. The crust forms the lightest and crispest of bubbles around a filling of finely minced meat scented with cumin and flavored with olives. Tongue vinaigrette is their equal, the thickish slices under a blanket of vinegared onions and green peppers. Then there is matambre, a handsome roll of beef studded with carrots, green beans and egg white so that it forms a mosaic when sliced. Served cold, it has a mild meaty flavor like a very delicate pate, perhaps not to everyone's taste but certainly appealing to everyone's eye. The appetizers also include shrimp as a cocktail or, better yet, sauteed in a creamy, butter-thickened garlic sauce; sweetbreads or kidneys saut,eed in wine; paprika-laced chorizos, Spanish sausages crisped on the grill; tomato stuffed with rice and tuna and a spinach pie that seemed excellently made but handled carelessly. Its crust one day was golden and crunchy and its inch-high spinach filling was layered with peas, diced ham and a whole cooked egg. But the taste was bland and faded, as if it had lost its flavor to the refrigerator.

You may have picked out your main course before you entered the restaurant, for the thick steaks and ropes of sausages grilling in the window are a strong draw. The waiter is likely to steer you to the filet mignon, and rightly so, for it is huge, thick and cooked as rare as you request, its surface crusty and tangy with a vinegar marinade. The porterhouse steak is much thinner, and thus emerges less crusty. Still its taste is smoky and tangy from the garlic-herb-vinegar sauce typically served with Argentine beef. Even more typical are short ribs, a cut of beef named churrasco and a collection of grilled meats called parrillada, again laced with that tangy garlic-herb marinade and sauce. The parrillada -- short ribs, kidneys, two kinds of sausage and sweetbreads -- is listed at $23.95 for two on the menu, but Las Pampas has been willing to serve single portions instead. Even for one it is enormous, three sweetbreads with a delicious crusty exterior and meltingly soft inside, two chunks of blood sausage, a chorizo that was dry but otherwise fine, three kidneys cooked just a touch too long and a large hunk of short ribs, all served on a sizzling platter over a flame. The taste was fine, all brushed with that vinegar-garlic chimichurri sauce, but everything swam in grease. All this feast needed was better draining and a few moments less of cooking.

The sleeper on the beef-oriented menu is the chicken. A whole chicken is boned, flattened and grilled so that the skin crisps and seals in the juices. It is a remarkable interplay of crunch and juiciness, chicken at its best. The rack of lamb could be its equal if it were better trimmed, for it had been marinated and grilled over coals, as rare and crusty as the filet, and sauced in a mustard-tarragon-wine-peppercorn mixture with lots of guts if not delicacy. Should grilled foods not tempt you, Las Pampas also serves prime rib and a combination of veal, beef and lamb medallions saut,eed and each in its own sauce. The meats are high quality and carefully cooked, but the sauces taste like bouillon, each different but none delicious. At Sunday brunch, the restaurant repeats several of these dishes plus homemade pastas.

Las Pampas does well in its accompaniments. The bread is homey and crusty; the green beans are fresh, crisp and buttery. And one suspects that if the thickly sliced fried potatoes were served immediately they would be less dry and dull. The wine list is short, understandably Latin in its emphasis and reasonably priced; a Chilean wine is available in carafe, and there are more half-bottles than most restaurants offer. As a refreshing aperitif there is white sangr,ia. Desserts are homey but far from outstanding.

In all, the food is pleasant and distinctive. Even more so are the surroundings, walls and tented ceiling of narrow, ruddily golden wood slats, with a ring of mirrors at eye level. The background music is sometimes Latin and occasionally the thumping of the upstairs disco. But whatever the accompaniment, Las Pampas has a comfortable and clubby feel. The waiters are helpful, willing to explain the dishes and make suggestions. One night the light dimmer was awry, so the waiter, in black tie, was climbing to each chandelier to unscrew half the bulbs. Atmosphere is important at Las Pampas. In fact, these days it seems that so are the service, the food --even the diner.