Try to remember. Did you lose your car keys today? Did you panic because you thought, aha, there it goes -- another brain cell zapped. Age! Senility! The End!

Stop a minute. Didn't you ever lose your car keys before?

How old were you? 20? 30? 16?

Did you think then that it was all over?

So what's different?

"One of the problems," says memory expert Robin L. West, psychology professor at the University of Florida, is that "people hear so much about the memory deficits of aging that they lock themselves into an inappropriate mind-set.

"They forget all the memory failures they had when they were young and they're not realistic about it."

In other words, they panic. Panic and anxiety, says West in her new book, "Memory Fitness Over 40" ($12.95, Triad Publishing), "when based on potential, but not actual, memory dysfunction can cause the very problem you fear: you will lose some of your memory ability because anxiety interferes dramatically with effective remembering . . ."

*One of the reasons West wrote the book was that wherever she went, as soon as her specialty became known, she would be besieged with questions from secretly terrified people who thought they detected signs of senility in their own memory lapses.

Her book is aimed "mainly at older people who are uptight about it" and designed "to help them relax and not be so worried about their memory."

Only in the past few years have medical scientists begun to grapple with the physiology -- the hardware, so to speak -- of memory. Researchers are trying, so far more or less unsuccessfully, to figure out where memory is stored and it is how retrieved.

Psychologists may approach it from somewhat different perspectives, but they, too, are becoming increasingly fascinated by the ease with which humans function daily -- knowing, for example, where they are in space, how to get to work, how to find the garlic powder in the grocery, where the science fiction books are in the bookstore.

Some memory functions do tend to slow down with aging, although many others do not, says West. Moreover, there are techniques for minimizing even the small deficits that may occur naturally. But these techniques or strategies need to be learned in advance -- at about 40, say -- because learning new things does become more difficult, or at least takes more time, as one ages.

Some other abilities that may change, West says:

*Thinking is slower. (Everything else is slower, too.)

*Paying attention and ignoring distractions become more difficult.

*More cues are needed to recall something previously learned.

But most memory functions remain stable throughout life. Here are some abilities that West and other researchers have found do not automatically change with age:

*Immediate memory. "You should be able to look up a telephone number, walk into the next room, and dial it just as easily at age 65 as you did at age 30."

*World knowledge. Also known as "semantic memory," this is the ability to remember specific information not tied to a time or event, as in the answers to "Trivial Pursuit" questions, the questions to "Jeopardy" answers, names of past presidents, baseball players' batting averages. Remembering how to ride a bike, swim or ice-skate also are unaffected.

*Susceptibility to interference. Everyone is susceptible, regardless of age. For example, if you meet 20 people early in the evening at one party and 20 more later on at another, the first names will interfere with your remembering the second group of names. This is true at any age and it doesn't grow worse as you grow older.

*The rate of forgetting. It may take longer to learn new things, but, writes West, "it is gratifying to know that your persistence will pay off. Once you have learned a new hobby, a new song or a new skill, you will retain it for as long as you could when you were young."

*Search process. The process of retrieving material stored in the memory banks, wherever and whatever they are, may slow down with age, but does not substantially change."

"One of the reasons we said '40' in the book title," says West, is "because I wanted people in middle years to start thinking now about positive memory change, not negative memory change, and what you can do now to improve your abilities. It is important to get really good at using memory strategies, because if you get proficient at them and use them consistently, they're not so likely to go away as you get older."

A positive self-concept is crucial, says West. Even today, when most specialists know that aging does not necessarily mean an automatic memory decline, she says, "the dominant view still emphasizes disability.

"When memory failures occur, no matter what the reason, they are usually attributed to aging if the person who makes the mistakes is older. If a young person makes the error, we say it was caused by memory overload ('I have too many things to think about right now'), failure to use appropriate memory aids ('I didn't write it down') or health problems ('I'm recovering from the flu')."

Indeed, health, memory overload and failure to use appropriate aids do contribute to memory failure in the young. But these factors also contribute to memory failure in older people, far more often than the aging itself.

West considers "Trivial Pursuit" a "great game," but even more, "a very good intellectual exercise." It's never been specifically tested, says West, "but research shows that older people's knowledge of public events, past celebrities and vocabulary are as good or better than younger people's, so that there should be no diminution of the competitive edge in such games."

*There is also some evidence, West says, "that reminiscing, thinking about old memories, can be emotionally beneficial -- and it is good practice in terms of retrieval."

West touches only lightly on the medical aspects of memory -- the physiology and neurology -- but other researchers, many of them involved with the study of Alzheimer's disease, are approaching memory from the inside out, working toward a drug that might prevent or stem the brain damage that occurs in victims of Alzheimer's and Alzheimer-like dementias. These afflict only a relatively small number of older people, but the relentless nature of their course, and especially the way these disorders gnaw away at the mind and memories of their victims, make them especially fearsome. And a breakthrough in the treatment of these disorders might well have eventual applications to memory enhancement in normal people, aging or not.

Dr. Trey Sunderland, a psychobiologist at the National Insitute of Mental Health, has had some preliminary success with a still-experimental drug called Deprenyl in a group of patients with early to moderate signs of Alzheimer's disease. Deprenyl is a "specialized MAO-inhibitor, now being used in the treatment for Parkinson's disease.

"I wouldn't want to raise any false hopes," Sunderland says, "but there has been some improvement in mood and some increase in social conversation with those in the group. We have not found so much that memory changes, but their families say they are 'better' and they do show mild improvement on some rating scales."

Researchers have found that a number of different brain chemicals are deficient in the brains of Alzheimer's victims. These include the neurotransmitters acetylcholine, serotonin, norepinephrine and dopamine -- substances that have a wholesale effect on mood, memory, coordination, energy and reflexes throughout the body. These substances are broken down and reabsorbed in the brain by an enzyme called monoamine oxidase-B (MAO-B). MAO-B normally increases in the brain as a person ages, but markedly increases in Alzheimer's patients.

This enzyme is blocked by the experimental drug Deprenyl. The NIMH hypothesis is that without the enzyme to break down the neurotransmitters, they will continue to function in the brain.

Sunderland and his colleagues are seeking persons in early to moderate stages of Alzheimer's disease or, as it is now being called, SDAT, for "senile dementia of the Alzheimer type." These patients will be given either Deprenyl or another drug, hydergine, which has been used for years to treat Alzheimer's. "It hasn't been well publicized," said Sunderland, "because no one knows why it works, and it doesn't show great changes, but of all the medicines used clinically, it is the only one known to show any improvement at all."

The NIMH trials will last from 12 to 18 months because the scientists believe that benefits will become increasingly apparent over a long term. More Information

Volunteers should phone social worker Sue Bell, 496-3421, or have physician send records to her at Room 3D41, Building 10, National Institute of Mental Health, National Institutes of Health, Bethesda, Md. 20892.