Circadian variation in suicide attempts in Tokyo, 1978-85, were analyzed on the basis of the Ambulance Service of Tokyo records. A significant circadian variation in attempts was demonstrated by the cosinor method, power spectrum analysis & analysis of variance. The peak time for attempts established by cosinor analysis was about 6:00 p.m. The circadian variation seemed to be associated with endogenous rhythms such as mood, as well as with daily variation in social activities. (19 refs.)