"He started with one assistant and two rats," people are fond of saying about Dr. Nathan Shock's human aging study.

Twenty-six years later, from his slightly cluttered office in what is today the four-story, $7.2 million Gerontology Research Center overlooking the Baltimore skyline, Shock dismisses the early history as untrue.

"In the beginning," he says, "we couldn't even afford the rats."

Today Shock's project, the National Institute of Aging's longitudinal study, is the nation's most comprehensive examination of what happens to the human body when it grows old. It involves more than 1,000 volunteers (roughly 650 men and 350 women) who have made a lifelong commitment to endure the pokes and probes of 143 NIA doctors and technicians for 2 1/2 days every two years to determine what the passage of time does to their bodies and minds. Volunteers have included teen-agers, construction workers, homemakers and a 96-year-old minister who traveled from Florida to participate.

"For years all we knew about aging came from patients, usually older people, who were already sick and in the hospital," says the 78-year-old Shock. "It was time to see what people out in the community were like."

Up to seven test subjects check into the center each week for the 2 1/2-day battery of examinations each undergoes every two years. They are tested to determine how vital organs such as the heart, liver and kidneys are functioning. They get chest X-rays and urine, blood and skin cell samples taken. Doctors probe eyesight, hearing and muscle strength and check the mineral content and amount of fat in the body. Psychological tests are also included.

"Unlike what most people think, there is no real peak time after which you start going downhill," says Shock. In fact, he maintains, if people would stop associating the aging process with disease, they might live a lot longer.

"The body is running at its best between the ages of around 30 and 45 -- even slightly longer for women," he says. "After that it just runs a little more slowly, not worse."

Some other findings:

* Heavy smokers have the lung capacity of a person about 10 years older than they are. Kick the habit, however, and within 18 to 24 months lung function will be back to near normal.

* Obesity is unhealthy, but a little extra weight might be beneficial. Standard height/weight tables are slightly too conservative, says the NIA.

* Physically active people over 30 have no greater muscle mass than nonactive people of the same age, and back and arm strength declines very little until about age 65. What changes as we get older is the willingness to stay with a strenuous task. It's not surprising that President Reagan is able to clear a lot of brush at his California ranch; the puzzle is why he still wants to at age 73.

* Certain age-related changes occur in the heart over time, but for the most part the ability to pump blood -- strength and speed -- is about the same at 80 as it is at 30.

* Elderly people who for years have been diagnosed as diabetic may simply have normal blood sugar levels for their age. Because older cells have a decreased sensitivity to insulin, sugar is removed from the blood more slowly. Also, older people have a lower amount of water in their systems, which means that they can take smaller drug doses than a younger person.

* Individual hand- and footprint patterns -- dermatoglyphics -- could be used one day to determine which people are at risk for various diseases. Breast cancer patients studied at NIA, for example, seem to have distinctly different prints than normal patients.

* Young and happy-go-lucky today probably means old and just as cavalier in the future. The same goes for moody and introspective. Personality doesn't seem to change with age. Also, people deal with stressful situations at 80 much as they did at 20.

* Physical appearance is a pretty good indicator of what kind of shape you're in.

Despite Shock's lifetime study, what causes us to grow old remains largely a mystery.

Some scientists believe that cells simply wear out and lose their ability to repair themselves. Others say that the genetic code provides an aging gene that switches on at an unknown time, or that the brain may release some kind of disruptive hormone.

According to the U.S. Census Bureau, American men will live 70.7 years, and women will live 78.3 years.

For now, the NIA doesn't discount that the "whys" of aging may someday be discovered, but so far questions about the "hows" have provided some curious results.

The rate at which individuals age, for example, varies widely from person to person. "When we first started this study," says Shock, "we really thought there would be some kind of common thread in the way people age , but there wasn't. It looks like people become more and more different as they get older and not more alike."

Two 20-year-olds are far more similar in test results, for example, than two 60-year-olds, simply because the older people have aged at different rates. Part of this wide variation has to do with heredity, something a person can't control. But another portion, something that can be controlled, is what Shock calls "clean livin'. It's just very hard to predict how a person is going to do down the line," he says.

The NIA's study is scheduled to continue indefinitely. Volunteers are still being added to the program, and after more than a quarter of a century, there remains much to discover: Why do some diseases appear only with the onset of old age? Why do women live longer than men? Perhaps in another 26 years more answers will have been found.

In the meantime, Shock says, waiting is the toughest part. "The frustrating thing about this kind of work," he laments, "is that it takes so damned long to find anything out." Shooting for 100

The way to a long and healthful life, say scientists at the National Institute on Aging, is still common sense. Aside from the questionable and unproven practice of mass vitamin ingestion, popular life-extension programs are really selling you nothing more than what you already knew, says gerontologist Nathan Shock.

It is impossible to predict how long an individual will live. But by staying on the straight and narrow, most people can have a shot at 100 if they:

* Eat less. Much attention in recent years has been given the kinds of foods we should or shouldn't be eating. But little has been said about how much we should eat.

Studies on animals show that if they're given about half what they want to eat a day, they live longer. The rate at which the body turns food into energy declines by about 3 percent every 10 years. As you get older you simply don't need to eat as much as you once did. At 50 a man needs 2,700 calories a day, a woman needs 2,000. After that, a man's need is 2,400, and a woman's is 1,800. If you keep eating the same amount, you get fat. And that's a strain on the body.

* Don't smoke. The U.S. surgeon general has determined that you're a nitwit if you don't know enough to stop by now. On average, a nonsmoker lives at least four years longer than a smoker. And, if you continue to smoke, you can pretty much pack in the idea of living 100 years considering the known risks of cancer and heart disease. "Low yield" cigarettes may not be the answer, though, because they often use ventilated filters and quick burning times to lower the nicotine counts. And while those methods work on the machines that measure cigarette toxins, they may not be as effective on real smokers.

* Exercise regularly. The funny thing about exercise is that there is absolutely no hard evidence it helps people live longer. It does, however, lower the risk of heart attack, burn calories and increase circulation. The NIA does not recommend jogging for older people because it puts added strain on already thinning bones and delicate joints. Swimming and walking are good alternatives.

Stay active. Take a cue from dance impresario Martha Graham at 90, or war-horse retired admiral Hyman Rickover, age 84. Mandatory retirement age seems to mark a milestone for many older people. They question their ability for intellectual skill since it seems they are no longer needed. That's wrong thinking, says the NIA. It's been proven that we have the capacity to learn well into the nineties, and as Shock points out, it doesn't help to "sitting in front of the idiot box all day." Join a church group, do volunteer work. Catch up on the classics you always wanted to read.

Living to 100 is not impossible, though the NIA stresses that good health at any age is more important than longevity.

As Shock and others see it, the real problem with following the path to old age is motivation. Many of the helpful methods demand a change of life style that most people don't feel like making. Moderation and motivation are the keys, they say, and you don't get those by reading life-extension books.