The tearful and tearless both cry on William Frey's shoulder.

Among those seeking Frey's help since he published "Crying: The Mystery of Tears" three years ago were a woman whose husband alternated bouts of tears and laughter, and a restaurateur whose cooks cried chopping onions.For the restaurant owner, the answer was easy and time-honored: Chop the onions under a mist of water.

Other times, it's more complicated.

Frey advised Anne Firebaugh, 64, who divides her year between Nan-oose Bay, British Columbia, and Alta Sierra, Calif., to take her husband Joseph, 71, to a neurologist. Mrs. Firebaugh said Frey's ideas reassured her belief - scorned by some doctors - that there was more to her husband's problems than runaway emotions. She learned that he is suffering from Lou Gehrig's disease.

Frey believes tears are a body's way of getting rid of chemicals produced by strong emotion.

"To get confirmation from someone who has made a study of this, that was very important to me," said Mrs. Firebaugh. "He is someone I can look to for help and guidance."

Frey, of the Dry Eye and Tear Research Center at St. Paul-Ramsey Medical Center, receives and answers letters nearly every week from people, including many men, concerned about tears or lack of them.

They get a sympathetic response because Frey knows the frustration of being unable to cry.

His mother suggested the line of inquiry when he was a graduate student in the early 1970s in search of a research project. He was spurred by curiosity over his own lack of emotional tears since he was a child.

"Once scientifically I started thinking about this, I started to realize this could be a very important process for us as humans. I became dissatisfied with what I had been satisfied, which was the fact I didn't cry very easily or very often," he said.

"So I went and talked to a psychologist. First trying to convince a psychologist that this is a problem worth dealing with is not easy."

But Frey eventually found someone who thought he had a worthwhile concern and helped him sort out his feelings. He estimates he now cries four times a month, even if his eyes only well up with tears.

"I hear things on the news or I read things in the paper or I hear something on National Public Radio or whatever. Whereas I used to not have any tears or cry, I will cry about it now, because I allow myself to do it."

The identities of the key substances removed in tears still elude him, partly because he may have made a conceptual error early in his experiments.

"I thought we could take emotional tears . . . and that we could compare those to tears that were produced by eye irritation. But the thing that I have come to realize lately is that when people tear in response to eye irritation, there is quite a bit of stress involved actually. When you're sitting there inhaling vapors from freshly cut onion, your eyes burn really a lot. You're feeling pain, stress."

His research backed that up, showing the same three biochemicals are present in both kinds of tears. They are leucine-enkephalin, an en-dorphin, and the hormones prolactin and Adrenocorticotropic hormone, known as ACTH. But he doesn't know in what amounts and is working on a way to determine that.

Tears also have a lot of material, such as wetting agents, that have interfered with his chemical methods of measurements.

"One thing we're interested in is trying to understand what is the biochemical basis of emotion and stress, what happens to us when we have changes in our emotions and our affect," Frey said.

"If we really understood the biochemistry of stress and emotion, we might be able to figure out what's going wrong in people who have emotional disorders."

So far though, his research has been confined to the tears of people who are sad, not the clinically depressed. Emotional tears are collected in the laboratory by showing poignant scenes from sad movies. "Brian's Song," "The Champ" and "All Mine To Give" have proven to be reliable tear-jerkers.

In his book, Frey wrote that Margaret Crepeau at the University of Pittsburgh School of Nursing discovered in 1979 that sufferers of ulcers and colitis, stress-related disorders, cry less often than healthy people.

"This suggests there's some kind of correlation between crying behavior and susceptibility to stress-related disorders," he said. "It doesn't mean that if you cry, you won't get an ulcer or anything like that."

The flip side of his research could benefit ophthalmology and sufferers of dry eyes, who number an estimated 2 million to 4 million in the United States, he said. Most sufferers of dry eyes are postmenopausal women.

Frey has found women cry on the average of five times a month, while men generally cry once a month. Other research has shown little difference in the regularity of emotional crying between boys and girls under the age of 12.

"That's very interesting because, for example, the hormone prolactin, which I think is probably involved in the sex difference (in crying) that we see between males and females, is at the same level in the blood of children," Frey said. "But it's 50 to 60 percent higher in the blood of adult women than in the blood of adult men."

In his book, Frey suggested research to learn whether there are anatomical differences in the tear glands of men and women. Other researchers have since reported finding those differences.

Pregnant women may have erroneously believed they cried more of-ten because they were more aware of their emotions during pregnancy, and they may have felt like crying more due to the added stress of pregnancy.

People who argue against his excretory theory of the tear gland generally point out that it lacks the filtering apparatus of the kidney and ask where the as-yet-unidentified waste substances come from.

As for the source of the unidentified substances being removed by the tear gland, he said, "I suspect that while it might come from the blood, that there is an equally good chance it might come from the brain. People forget there's a lot of things still not known about anatomy."