It might be called the Great Tomato Seed Caper.

As you read this sentence, some 12.5 million tomato seeds are orbiting in space in the Long Duration Exposure Facility, a NASA experiment cruising 300 miles above the earth's surface at a speed of 17,000 mph.

The ultimate purpose: to determine the effects of long-term radiation exposure on living organisms and to see if food can one day be grown in space.

It is one of several dozen experiments that have already flown aboard the space shuttle or will in the future. They represent a new dimension of two classic scientific pursuits -- biology and medicine -- now poised to take some giant steps for science in the inhospitable environment of earth orbit.

Space biology "is in its real infant stage," says Ron White, life science program scientist at the space agency's headquarters in Washington. "We're still very limited to sending things up and seeing what happens." It's about the equivalent, White says, of "putting things in a box on earth, leaving them on the laboratory bench and seeing what happens."

But scientists don't expect these new fields to remain in their infancy for long. The zero gravity of space, combined with the shuttle's high technology, will enable researchers to produce far more sophisticated experiments probing diverse questions -- ranging from how the brain functions to whether it may be possible to grow food in space.

Today, many of the experiments are "aimed at providing a safe and healthful environment in space" for current astronauts and future space travelers, says Chris Schatte, a project scientist with NASA's Ames Research Center. "We have no reason to believe that shuttle missions are not safe. There's more concern about the space stations and 90-day stays up there." The last three Soviet cosmonauts had a "longer period of adaptation" since returning from their record 237-day mission, says Dr. Arnauld Nicogossian, director of life sciences for NASA. "They were able to recover 40 to 50 percent of their preflight activity at the end of the first week back," Nicogossian says. "They recovered 75 to 80 percent by the end of three to four weeks," but overall, it took about two months for the cosmonauts to fully recover from their mission.

By comparison, the American astronauts who flew a 90-day mission in Skylab took only about three to four weeks to regain their preflight activity level.

Most space medicine and biology experiments focus on two main questions:

How do people, organisms, plants and other biological systems behave without gravity?

Does the potential for greater exposure to radiation in space pose any harm?

The zero-gravity environment offers researchers a textbook opportunity to study what effects gravity exerts on biological systems on earth. (In science, to truly investigate the effects a physical force has on an organism, that force must be removed.) "Everything on earth has evolved in gravity," says NASA's White. "Whether we're dealing with the cell or all of man, we don't know what effect gravity has. We're beginning to probe."

In one early experiment aboard the shuttle, researchers germinated seeds to see if they would know which way was up. To the scientists' surprise, the plants did -- they still grew away from the floor of the shuttle.

Another experiment examined the effect of zero gravity on immunoglobulins -- proteins that target invading germs for destruction by the body's white blood cells. "There were no profound changes in space with immunoglobulins," reports Mel Buderer, a physiologist and project scientist at NASA's Johnson Space Center in Houston. But when an astronaut tested the activity of human white blood cells in space, there was a "90 percent decrease in response," he says. Whether decreased immune function may be a side effect of space travel is one question researchers hope to answer.

One area where space medicine may provide promising insight is in brain research. "We're getting a better understanding of neurology and how we're put together," says NASA's Buderer.

New information is expected to emerge about the neurovestibular system, which conveys information from the inner ear to the brain about the body's posture and movement and helps regulate coordination and balance. This is the system that seems to malfunction in people who experience space sickness.

"There are little crystals in the head, which are sensitive to orientation," explains White. "What feedback that could have in space sickness one could only speculate, but we may get a much better understanding."

Another health area space medicine may help scientists understand is the cardiovascular system. Without gravity to pull blood into the legs and lower extremities, body fluid moves up towards the head. That's why astronauts sometimes look like they have puffy faces in space.

Studies in space also have shown that in zero gravity the body excretes some of this extra water which on earth would accumulate in the legs. During reentry, gravity pulls blood and other fluids to the lower regions of the body. But this can leave less blood for the brain and heart and result in lightheadedness and sometimes fainting.

Today, as a result of research and experience, astronauts can prevent this problem by wearing special suits that constrict blood flow around the middle of the body during reentry and thus prevent blood from pooling too fast in the legs. Another technique is to replace some of the lost fluids by drinking salted water within a few hours of landing. Upcoming experiments aboard the shuttle will examine the cardiovascular system in humans, monkeys and rats.

"One experiment -- which is kind of unique -- will measure peripheral blood flow in rats," says NASA's Schatte. Using a special technique developed by a North Carolina researcher, scientists "will view individual red blood cells moving through arterioles" -- tiny blood vessels -- and "see in detail how circulation is behaving."

Another experiment -- this time a private venture by the McDonnell Douglas Corp. -- tests the potential for manufacturing drugs in space. Using a process called electrophoresis, which separates varying sizes of molecules based on their response to an electric charge, a McDonnell Douglas researcher is attempting to separate a hormone that could then be used on earth. Exactly which hormone and how it will be used is yet to be announced by McDonnell Douglas, although a spokeswoman for the company said that clinical trials will begin shortly.

Since some of the changes in space resemble diseases found on earth, space medicine and biology may also hold clues to such diverse earthly illnesses as the complications of chronic bed rest and the problems of osteoporosis -- the severe calcium loss that leads to brittle bones.

"We're looking at temperature regulation in space," says Schatte. "We've not identified a human problem. But the preliminary data from other flights is that the rhythm of body temperature falls apart. We're putting skin sensors on animals, and we'll see if desynchronization takes place. There are probably a couple of centers in the brain that control such things as heart rate and body temperature. Now there's some good evidence that if there's desychronization, that's one of the first steps toward ill health."

"The ultimate aim," Schatte says, "is to find out when the animal or person might be going to get sick. It's a first step."

But like all areas of science, space medicine and space biology are often hampered by setbacks. The first electrophoresis experiment was contaminated in space, a problem that now seems to be solved.

Space science is also plagued by the vagaries of shuttle scheduling problems. The tomato seed experiment was originally a one-year study. But because of mechanical difficulties with the shuttle, the seeds won't return until August 1986 -- some 2 1/2 years after they left earth. This gives the seeds the distinction of "having the longest exposure in space of any life form," says NASA biologist Doris Grigsby, project manager for the experiment.

Following their return from orbit, the tomato seeds will be distributed to 150,000 classrooms, where students of all ages will become "principal investigators" for NASA. Youngsters in elementary school will attempt to grow the seeds, while those in graduate school will analyze the chromosomes -- strands of DNA that may be altered by radiation after such a long stay in space.

How the seeds fare after their stay in space is important knowledge not only for growing food in space someday, Grigsby says, but also for "one day being applied to humans."