A 66-year-old patient with 48-hour unipolar-depressive cycles was studied in the hospital for one month and in an isolation unit for two weeks. At 3-hour intervals during the day and once at night the mood state was assessed by two self-rating scales and urine was collected for the determination of urinary free cortisol (UFC). In the hospital there was a regular alternation between 'good' and 'bad' days. Without exception the switch from normal to depressive mood occurred between 22:00 and 2:30, when the patient was sleeping. The switch from depressive to good mood was variable and occurred in the late morning or in the afternoon. On a 24-hour basis a highly significant correlation was seen between the mood state and the UFC excretion: r = 0.73, P less than 0.001 (hospital); r = 0.82, P less than 0.01 (isolation unit). The shape of the 24-hour profile for the UFC excretion was similar on good and bad days, with a maximum between 2:30 and 7:00 and a minimum between 19:00 and 2:30. However, on bad days UFC was elevated at all clock times. A decrease in adrenal cortical activity preceded or was parallel to substantial improvement in the mood state. Internal desynchronization occurred in the isolation unit but not in the hospital, with a rest-activity period of 18.6 h and a period close to 24 h for UFC excretion. Changes in mood were associated with both cycles, and a very close link between the switch from normal to depressive mood and the sleep state could be demonstrated.