"The Rescuers," opening today at seven theaters and three drive-ins in the Washington suburbs, is one of the most rousing and appealing animated features ever made by the Disney studio. The last production for several members of the original feature animation unit assembled by Walt Disney in the late '30s, the film is both a triumphant swan song and gladdening act of regeneration. "The Rescuers" is certainly the best Disney feature since "Mary Poppins," and it's also the first conceived since the death of the founder to offer abundant assurance that the Disney tradition is capable of sustaining itself for another filmmaking generation.

The scenario for this swift, amusing and unexpectedly thrilling entertainment was inspired by characters and situations from two children's books, "The Rescuers" and "Miss Bianca," by the English writer Margery Sharp, who also enriched the movies with "Cluny Brown."

A rescue aid society operated by mice dispatches two agents - prim, upperclass Miss Bianca and shy, stalwart, plebian Bernard - to find a missing orphan who has inserted a message of distress in a bottle that washes up in New York harbor.

Undaunted by numerous obstacles, particularly the greedy, flaming-haired villainess, Madame Medusa, who is holding the orphan, a little girl named Penny, captive, the intrepid mice presevere until their mission of mercy is accomplished.

At 76 minutes, the film is almost nonstop slapstick and medodramatic action once the initial, preparatory exposition is disposed of. The Disney live-action comedies of recent years have tended to be laborious and lumpy. "The Rescuers" achieves a pace and tone much closer to the snappy, zingy consistency of the old Disney cartoons.

It's also closer to the consistency of "Star Wars," which demonstrates how to be old-fashioned in a new-fangled way and has essentially the same plot. As a matter of fact, "Star Wars" could be called "The Rescuers" without distorting its plot in the least. The two movies even share similar action highlights and set-pieces.

For example, the heroes of "Star Wars" get trapped in a boggy garbage pit whose walls threaten to crush them. In "The Rescuers" Penny, Miss Bianca and Bernard struggle to escape from an underground cave filling up with torrents of water as the tide rushes in. One of the most beguiling characters in the show, a dragonfly called Evinrude who works himself into tonge-lolling exhaustion imitating an outboard motor, is set upon by hungry bats in a chase sequence reminiscent of the dogfights between spaceships in "Star Wars," which, of course, employed animators to achieve certain indispensable thrills and illusions.

Another index to the increased level of stimulation and excitement in this new Disney picture is the frequency with which the littlest kids in the audience were frightened at the screening I attended. My wife was running a ferry service during the climactic sequences, because our 3-year-old daughter was demonstrably fascinated but scared by the action on the screen.

For what it's worth, our 6-year-old daughter was just fascinated. The primary source of concern to our younger child was evidently the character of the villainess rather than the perilous situations. The flamboyant caricature that makes Madame Medusa an irresistibly funny, campy menace to older children or adults may seem downright threatening to the youngest kids.

Medusa has been brilliantly animated, particularly her entrances, which are uproariously explosive, and the saggy floppiness of her body as indicated by the bulges and waves that come and go through her clothing. Her lines are also brilliantly spoken by Geraldine page, who gives Medusa exactly the right note of extravagant, haughty agitation.

The characterization is especially strong, all the way from Evinrude to the humans like Penny, Medusa and Medusa's ineffectual henchman Snoops, and it provides a solid foundation for comic, melodramatic or sentimental embellishment. Penny, for example, seems a much more courageous and resourceful kid heroine than the Little Orphan Annie of the musical "Annie." Even her status as an orphan is exploited for more incisive and affecting interjections of sentiment.

The voices chosen to read the characters are also consistently pleasing. It's surprising and amusing to hear how well Bob Newhart and Eva Gabor harmonize as Bernard and Miss Bianca or Geraldine Page and the late Joe Flynn as Medusa and Snoops. Perhaps most satisfying of all is the sound of Jim Jordan, old "Fibber McGee" himself, in the delightful role of Orville the albatross, whose efforts to land and take off seem to have inspired the animators to their finest moments of sustained slapstick.