The aim of this study is to evaluate the presence of a daily chronobiologic rhythm in suicidal behavior and opiate overdose syndrome. One hundred and eighty-one patients and 117 subjects, respectively, were investigated during one solar year. In attempted suicide a significant circadian rhythmicity was found for patients as a whole and females separately, while in the latter study significant periodicity concerned the total subjects and males singly. Both situations disclosed a peak of incidence in the evening, at about 7 p.m. Potential explanations of these results include diurnal variation in mood, possibly related to cyclical hormone and neurotransmitter secretions, and social habits.