Another year, another nuclear war. In 1983, ABC's "The Day After" was seen by an estimated 100 million people, considerably more than will see "Testament" on the season premiere of the PBS "American Playhouse" series at 9 tonight on Channel 26. Instead of attempting to portray the mass effects of a nuclear strike, this film deals with one family in a northern California town that lingers and dies under fallout from bombs.

The movie, monumentally depressing, was released to theaters one year ago and attracted praise from some movie critics, a few of whom were anxious to trash ABC's "Day After" in the process, even though "Testament" was itself conceived as a television show. In fact, "The Day After" was as good a film as "Testament" is, if not as artily posed. It took more courage for ABC to put "Day After" on than it does for PBS to air "Testament," and history will show, if there is any history, that "Day After" had incomparably more impact.

"Testament" has no political content. It doesn't detail any of the incidents that led up to World War III but instead concentrates on the Wetherlys, a family of five living in casual affluence and isolated comfort in a tidy suburban community. Writer John Sacret Young made the children awfully lethargic and cranky, but otherwise the details of family life ring moderately true. One day while dad is off in the city the world as they have all known it comes to an end.

The film, directed by Lynne Littman, probably works better on the TV screen than it did on the movie screen, since the emphasis is on intimate tragedy rather than wholesale disaster. There are no pyrotechnical special effects except for a brief blinding flash. The town is leveled in spirit but the buildings remain standing. Slowly, of course, people will get terribly sick from radiation poisoning and die. A baby is the first to go.

Could this be the way it might happen to such a town? It seems oddly plausible that people would continue to put their trash in Hefty bags and pile it on the curb even if the collectors never do come to pick it up. There are long lines at the gas station and armed guards standing outside the supermarket, scenes reminiscent of "Alas, Babylon," the doomsday drama CBS presented on "Playhouse 90" roughly a quarter of a century ago. Eventually the town cemetery fills up and bodies are burned in fields. But old Mr. Abhart, played by veteran character actor Leon Ames, sits at his ham radio trying to make contact and declaring, "I'm not giving up on this old world yet."

Jane Alexander plays the mother whose stamina and suffering are the focus of the film. Naturally, as in virtually every role she plays, Alexander is so noble you wonder that people don't turn to stone just from looking at her. The performance is stony itself until the last half hour, when the mother sits in a chair cradling the little boy that will be the first of her children to die. From that point on there is virtually no defense; one is going to be moved by the iconography of this picture no matter how much resistance is put up.

Others in the cast include Roxana Zal, the Amelia of ABC's "Something About Amelia," as the Wetherlys' daughter Mary; Rebecca De Mornay, later the crafty hooker in "Risky Business," as the mother of the town's first radiation victim; and Philip Anglim, "The Elephant Man" on Broadway and TV, as a local priest at a loss for encouraging words. As Alexander's husband, William Devane is only in the film for a short time, which is just as well.

The two best performances are by children: Ross Harris as Brad Wetherly, a teen-ager forced to mature overnight by the crisis around him, and Gerry Murillo as Hiroshi, a retarded Japanese boy. Murillo is in fact retarded himself, but that doesn't diminish the superb job that he does. His oblivious, hopeful smile is sustenance through grim tribulation. He also functions, of course, as a symbol of Hiroshima and the start of the nuclear age.

Naturally PBS couldn't present this movie without doing something deplorable. Before it begins, PBS hypes it with the kind of jangly promo-tease the commercial networks use -- the legacy of former PBS president Lawrence Grossman. An announcer hails it as an "exceptionally powerful and eloquent story of one family's love and courage when faced with the ultimate horror" that "has been called astonishing, shattering and utterly unforgettable!" And then there's a parental warning that attempts to exploit the controversy generated last year with "The Day After." Quel bilge.

The film itself survives the cheaply gaudy PBS wrapper. It also survives some of its own heavy-handedness. The town was named Hamelin so that a parallel could be drawn with a kiddie production of "The Pied Piper" that goes on even after the bombs have hit. In the pageant, a child playing the piper says, "Your children are not dead. They will return. They are just waiting until the world deserves them." No, "Testament" is not subtle, but then, neither is its subject.