Is there life after Doonesbury? This is the question millions of hungry fans have been asking themselves all through 1983 while cartoonist Garry Trudeau takes a sabbatical from his popular comic strip.

It is also a question raised by Trudeau's new musical show, written in collaboration with composer Elizabeth Swados, which is finishing its run at Boston's Wilbur Theatre and preparing to move to Broadway this week.

In "Doonesbury"--the musical--the comic strip characters finally grow up and receive an eviction notice from the communal home they have called Walden. Mike Doonesbury is engaged, Megaphone Mark gets a radio job with a regular pay check, B.D. is hired by the Dallas Cowboys, and the whole crew finally graduates. Zonker Harris is carried bodily up to the platform where he is handed his diploma, screaming loudly and accurately, "I'm not ready."

Finally, the question is being faced by composer Swados, self-described as an "artistic general practitioner," who has been living with the show since its early planning stages and has lately been going through the hectic routines of a pre-Broadway tryout. Swados is sure that there is life after Doonesbury and is eager to plunge into it.

"I will leave the show in about aweek," the multi-talented 32-year-old said at a late breakfast in a Boston restaurant a few days ago. She had been up until 4 a.m. after the previous night's performance, conferring on last minute changes, and was ready to run off again to an early rehearsal.

"I hope I'll get back to my novel," she said, "that's what I need to do first."

It will be her second novel; the first, "Leah and Lazar," was published last year with good reviews. She had worked on it for six years, while also turning out musicals, television productions, a ballet, sound tracks, and incidental music for theatrical classics by playwrights who range from Sophocles to Bertolt Brecht. Her second novel is called "Under the Influence," and that's about all she wants to say about it at the moment.

"I have a whole draft waiting to be chopped to pieces," she says. "Then I'm working on a two-hour, prime-time television special based on the Pied Piper, which is one of my favorite jobs I've ever had because the way I learned about opera and music and theater up in Buffalo was by seeing 'Amahl and the Night Visitors' and 'Peter Pan' and 'The Wizard of Oz' on TV. I think that form is still very viable for getting interesting stuff to young kids--particularly in cities where live performances don't have a high priority. That's going to be fun, and that's in its third draft. It's going to be shot so I have to finish it up. Then I have an oratorio--I have a lot of things going on right now."

The idea for the oratorio came to her last year during a trip to Jerusalem. It will be about all the different religions that exist in that city, and it will focus on the sounds of religion--the various languages and musical styles, the rhythms and melodic patterns used in their worship services. "The uncanny thing," she says, "is how similar a Muslim's prayer is to a Sephardic prayer or a Russian Orthodox. Everyone who is participating in it is finding curious similarities.

"The oratorio's point of view is that the prayers become more and more fanatic in the backup chorus and the orchestra, so that one feels there is both great beauty and great danger in this kind of dedication."

It is scheduled to have its first performance in Rome next spring. Meanwhile, Swados is also working on another Broadway musical based on the story of Jonah and the Whale.

For "Doonesbury," as for the Jerusalem oratorio, Swados has explored the deeper harmonies in a variety of musical styles--basically the kind of music that would have been heard in the Walden commune. Musically and theatrically, a large part of the show's box-office potential seems based on the theory that the world is ready for 1970s nostalgia. Still, the script is periodically updated with topical references to the Reagan administration.

In one recently inserted interlude, the stage is filled with a drop showing the familiar Trudeau cartoon of the White House, while dialogue--with one voice very much like that of Ronald Reagan--comes out of the loud speakers. The president is conferring with an adviser about trouble with Canada, which has been complaining about acid rain.

"Could we invade?" he asks.

"I don't see why not," says the adviser. "Are there any American lives that need safeguarding?"

But the basic atmosphere of the show, firmly established in the songs and the dances, is that of an earlier era--the age of communes and flower children whose passing is symbolized when Zonker reluctantly takes his diploma. The musical styles range from rock to a James Taylor sort of soft pop, which is used when the music is given to feckless idealist Mike Doonesbury.

For Uncle Duke, a commanding presence in the show, the variety of styles ranges from a sort of talking blues to a taste of Eisenhower-era do-wop in a number called "It's the Right Time to Be Rich."

Zonker's big number is "I Came to Tan," and during it, the lighting is manipulated to make him acquire an instant tan on stage while the audience watches and two sportscasters comment: "Harris has been plagued in the past by unevenness and peeling."

But perhaps his most memorable musical moment is an interlude with a gospel-style chorus of four plants he has been watering--singing puppets that look like a new, vegetable species of Muppets.

"I remember the first time the plants came out," Swados says. "I turned to the choreographer and said, 'This is what we do for a living?' "

It is not pure pleasure, of course.

"There was a song that I think was one of the best in the show," Swados says, "but it just didn't do anything to push the show forward. Sometimes, if you write something that's good on its own terms like that, it's like a very egotistical performance--you take it and put it in a one-person show and it would work, but in the middle of a lot of other things, it would just slow everything down. It never stops hurting--but maybe you can use it somewhere else."

Most of the time, Swados does her own words as well as the music, but in the case of Doonesbury, Garry Trudeau had an unquestionable proprietary claim. You might think that she would have complete control when she is writing a novel, but she says that is not necessarily true either:

"A novel is a real visit to another world. You travel into the rhythms and the sounds and the colors and the language of other people and work very hard for a very long time to get to know them and then sort of see what they're going to do. Then you have goals and situations that you want them to be in--but sometimes the truth of them won't let you put them in situations that they couldn't be in. It's an all-involving process."

She "would rather not say" what her next novel is about. "The whole first draft is done," she says, "but you never know what's going to happen."