The Burger War is sizzling again.

Burger King, which opened the hostilities a year ago with a controversial ad campaign touting its hamburgers as tasting better than those fried by rivals McDonald's and Wendy's, is launching an ad campaign claiming that millions of fast-food fanatics are switching to Burger King.

And Wendy's is introducing an ad campaign that chides its competitors for using frozen hamburger patties and forcing customers to "step aside" for special orders.

McDonald's, meanwhile, is keeping its Big Mac shut. The industry leader says it doesn't believe in knocking the competition, and McDonald's officials say they believe their burger's quality speaks for itself.

The battle for America's drive-in taste buds has high stakes. Fast-food sales are expected to reach $41 billion this year. And the traditional fare of burgers, fries and shakes is no longer enough--seeking to broaden their appeal, the fast-food chains have added a variety of other items to their menus.

McDonald's says that Chicken McNuggets have been more successful than the company had expected; Burger King is pushing its salad bar with pita bread; and Wendy's is quickly moving a stuffed baked potato into national distribution.

But hamburgers are still the industry's bread and butter, and they are the focus of the battle of the fast-food giants.

Burger King, a division of Pillsbury Co., has been the most aggressive of the three industry leaders. The Miami-based chain riled its rivals with its taste-test and its broiling vs. frying cam- paign in the past year, although it had to back off when its competitors filed lawsuits.

Burger King's new campaign isn't likely to go down easily, either. The $40 million effort will urge consumers to move from the Golden Arches to the home of the Whopper.

The ads will show studies that indicate that millions of consumers have already made the switch. One ad will feature a family named--ahem--MacDonald, who say, "Tell 'em the MacDonalds sent you." The ads also mention Burger King's change of soft drink brand to Pepsi Cola from Coca Cola.

Wendy's officials say they don't believe in mentioning competitors by name in their advertising. But they don't mind hinting--one of Wendy's new commercials says, "Two famous hamburger places use frozen hamburgers." The television commercials, produced by the same company that does Federal Express ads, share that famous campaign's humorous style.

One commercial, entitled "Back There," depicts the other burger chains storing cooked food, seemingly indefinitely, in a back room.

Another shows a customer at an unnamed competitor being told to "step aside" with his special order while other customers are served.

"It's advertising that attempts to talk about a basic difference," William M. Welter, senior vice president of marketing for Columbus, Ohio-based Wendy's, said in an interview. "Burger King's advertising has been based on opinion and surveys and innuendo. Ours is based really on hard facts, or more on the system itself."

At McDonald's, officials say they don't plan to change their advertising to respond.

"You're not going to see that kind of stuff coming from us," says a McDonald's spokesman. "We're going to respond by continuing to do what we're doing and doing it better."

And even as he prepares a new salvo, Welter believes the Burger War will eventually play itself out.

"We don't want to end up spending all of our time worrying about the other guys," he said. "That's the kiss of death . . . you have to worry about the consumer."