We investigated circadian rhythms of body temperature in 62 inpatients with major depressive episodes, by monitoring the deep body temperature through the abdominal skin every two hours for a consecutive 48-h period. The data were analyzed by both the least-squares method and the maximum entropy spectral analysis (MEM) and were compared with those in 29 normal volunteers who apparently had a regular 24-h sleep-wake schedule. Circadian rhythm phase disturbances in the depressed patients were likely to be manifested in a phase normal or a phase delay pattern rather than in a phase advance pattern. The amplitude of body temperature was significantly smaller and the mesor was higher in the depressed patients than in the normal subjects. Analysis by MEM revealed that the periods of circadian rhythm of body temperature tended to be longer in the depressed patients than in the normal subjects, though there was no significant difference. The power spectral density by MEM was significantly lower, and there were significantly more ultradian rhythm components in the depressed patients than in the normal subjects. These findings suggest that the fundamental rhythm disturbance in depression may be a weakening of the coupling processes between internal pacemakers and an abnormal sensitivity to environmental information.