Let's grab our rackets and go play . . . wallyball?

If wallyball doesn't grow into a national sport, it won't be Joe Garcia's fault. Mr. Garcia, inventor of the three-year-old mixture of racquetball and volleyball, spends his days criss-crossing the United States promoting his indoor game. About 800 racquetball clubs have enlisted in his cause to date.

Although the court game started in the Los Angeles vicinity, it has made its best mark east of the Mississippi River, particularly in Illinois, Pennsylvania, New Jersey, Ohio, Virginia, Connecticut, and Massachusetts, Garcia says.

''I don't think of it as a seasonal game, but maybe it is,'' says the southern Californian. ''On the East Coast, people have eight months of winter. In California, it is hard to get people off the beaches and inside.''

Wallyball is played by two-, three-, and four-person teams on a racquetball court with a net strung from wall to wall. Volleyball rules are followed, except that the ball can be played off the surrounding four walls.

The blue, hollow sphere batted to and fro is the size of a volleyball, but made of the same material as a racquetball.

Racquetball club owners and managers like it because the low-overhead game rings up playing time and business on otherwise empty courts. Garcia emphasizes a different selling point.

''Both sexes can play it, and play it well together,'' he says. ''One sex doesn't overpower the other.''

The majority of wallyball participants are volleyball players, Garcia says.

''I don't try to convert racquetball players,'' he said, ''but it is starting to catch on with them. It's like a new food. You see it, but it takes awhile for you to try it.''

Apparently people are nibbling at the hybrid. A Columbus, Ga., racquetball club drew 3,000 curious spectators to a tournament last summer.

The University of Minnesota's Duluth branch runs intramural wallyball leagues and teaches it in accredited physical education courses.

A Braintree, Mass. racquetball club started the newcomer on two courts recently, and quickly added two more courts to meet demand.

For Garcia, manager of a racquetball club in Calabasas, Calif., this is only the tip of the iceberg.

The game was created in a moment of boredom. Garcia and several of his workers strung a rope across a court one day when work was slow and slammed a racquetball back and forth.

''One thing led to another and somebody said, 'Let's call it wallyball,' '' Garcia says.

''I'd like to think I'm the first to come up with the game, but as I travel around, I'm finding that a lot of other people, some as far back as 20 years ago , also thought up the game. I'm the first to promote it.''

Garcia, following the advice of a friend who counseled him to ''go for it,'' registered the wallyball name and formed a corporation to advance the new sport.

Later, at a racquetball trade show booth in Los Angeles, Garcia met Lee Jones , an employee of AMF Voit.

''Lee came up to me and said, 'We can make you a better ball,' '' Garcia says. ''I scoffed at him and said, 'Nah, volleyball players like to use leather balls, it's what they're used to.' ''

But several weeks later Jones approached Garcia with a specially made ball. After testing and refinements, the blue wallyball was born.

Garcia has since signed a licensing agreement with Voit, making the sporting goods manufacturer the sole maker and distributor of wallyball equipment.

A native of Florida, Garcia, was an all-state high school catcher before signing with the Los Angeles Dodgers and being assigned to the Dodgers' Miami affiliate in a rookie league.

However, a common affliction - the inability to hit a curve ball - ended Garcia's professional baseball career. After a stint in the Air Force, Garcia settled in California to pursue an acting career.

He landed parts in the film ''China Syndrome'' starring Jane Fonda, and televison shows ''Three's Company'' and ''Policewoman.''

But he hopes wallyball is the thing for which he will be remembered.