GA Technologies, a high-tech firm in California, wants the Defense Department to know that it can offer the latest model in high-energy beams, just right for zapping nuclear missiles in flight.

In case the Pentagon is interested, though, GA Technologies also happens to have an antidote: the latest word in "survivable materials" to prevent those missiles from being zapped.

If building a defense against strategic nuclear missiles will be big business, then the business opportunities if the Soviet Union responds in kind will be no less exciting.

It is a game as old as warfare -- the development of new generations of weapons, and the development of countermeasures to those weapons, and of counter-countermeasures and onward, as long as the budgets last. Even before President Reagan launched his Strategic Defense Initiative (SDI), the Pentagon was intrigued by "penaids": new technologies to help nuclear warheads penetrate an enemy's defenses.

With the advent of "Star Wars," though, "penaids" will become increasingly important, both to defeat any defenses the Soviet Union may build and to teach Star Wars contractors what they must overcome. In the upside-down world of nuclear weaponry, there is no irony intended when adjacent labs in the same arms company work simultaneously on both new weapons and countermeasures to them.

Thus, at the Air Force Association weapons fair last month, McDonnell Douglas Co. boasted on one curtained wall of an exhibit booth of its Star Wars prowess while displaying -- with equal pride -- on a facing wall its Star Wars countermeasures.

The company touted its MaRV, for example, a maneuvering reentry vehicle that would dodge an enemy's defenses and "enhance penetration and accuracy of ICBM forces." Even more alluring was the DSV, or defense suppression vehicle, which would hitch a ride on an intercontinental missile and then, wiggling through enemy defenses, home in on enemy radars so that follow-on warheads could drop in unimpeded.

A committee of the Electronic Industries Association predicted last year that, for years to come, countermeasures will offer a "growth market . . . rich in electronic content."

Said the panel: "We rate penaids, including smart end-game decoys, as good bets."