434 bipolar manic-depressive patients were followed longitudinally. The course of the disease changed in many patients over the years. 67 cases became rapid cyclers (two or more cycles per year); in 40 of these cases (12 men and 28 women) the change of the course took place after intense or protracted use of antidepressant drugs. In their previous course these patients had not received antidepressant drugs. The common feature of the transformation of the previous course to a continuous circular one was the appearance for the first time in the course of the disease of hypomanic episodes after the depressions, or the accentuation of hypomanias that had been of milder intensity in previous recurrences. The patients who developed continuous circularity under antidepressant drug treatment were of highly energetic temperament. The hypothesis is advanced that these patients have latent hypomanias, which become clinically manifest under the action of antidepressants. The intensification of an underlying hypomanic process by the antidepressants would precipitate another depression and establish continuous circularity. The change to a rapid cyclical course was more frequent in middle-aged patients and in women.