"She has two different sorts of mood," the Greek poet Semonides wrote of his wife in the 6th Century B.C. "One day she is all smiles and happiness. . .Then, another day, there'll be no living with her. . .she flies into a rage. . ."

Some 2,600 years later, both men and women are still struggling to cope with these Jekyll-and-Hyde female mood swings that may be the most commonly reported symptom of Premenstrual Syndrome, or PMS. Although theories abound as to why some women experience distressing physical and emotional changes during the two weeks prior to menstruation, there is still no fully adequate explanation. But while scientists don't know precisely what causes PMS, they do know what doesn't.

"PMS is not caused by 'raging hormones' and it is not 'something in the woman's head,'" write Drs. Samuel Smith and Isaac Schiff in their new medical text, "Modern Management of Premenstrual Syndrome." "PMS is a reproductive endocrine disorder" in which a woman's hormones "interact with brain neurochemicals, prostaglandins and other body systems differently." Severe PMS affects surprisingly few women, an estimated 3 to 5 percent. Less intense monthly distress is much more common, affecting about 30 to 40 percent of reproductive-age women. PMS is more common after age 30 and can be especially troublesome in the perimenopausal years as a woman's hormones shift towards menopause.

One of the most puzzling questions about PMS is why some women have very mild symptoms and others take a monthly nose dive. "The answer is not entirely clear, but probably relates to hypothalamic maturity, exercise and body fat," says Jerilynn C. Prior, professor of endocrinology at the University of British Columbia.