A 500-pound Kodiak bear and her two 200-pound cubs escaped from their cages at the National Zoo yesterday and spent about 20 minutes in a grassy area separated from the public only by a four-foot-high chain link fence.

As two policemen armed with rifles and a veterinarian with a tranquilizer gun stood by, the bears, which the zoo lists among its dangerous species, were lured back to their cages by keepers with food.

Zoo officials said the three brown bears escaped from their cages about 12:25 p.m. by working free the catch on one door and forcing open a second that was partly shut.

Normally the bears may enter and leave their cages only through a front door, which gives access to an outdoor area separated from the public both by a fence and by a deep trench that is considered impassable.

While the bears frolicked merrily yesterday in the rear area, which has no trench, there was concern, according to Daryl Boness, curator of mammals, that they might climb the fence that stood between them and the public.

However, the mixture of ground beef and vitamins used by keepers Bill Rose and Robert Douglas, apparently proved a stronger lure to the errant bears than the prospects of a stroll in the December sunshine on Connecticut Avenue, and the animals returned to their cages.