Forty-six patients with various forms of endogenous depression were treated by night sleep deprivation (NSD) which was used as the cardinal method of treatment. A single session of NSD induced a considerable clinical improvement lasting 2-3 days in the majority of patients. Repetition of the procedure (3-6 times with a 2-3 day interval) led to a significant and stable clinical improvement in 32 of the 46 patients. The highest effect of treatment was observed in such symptoms as depression and suicidal tendencies, whereas in patients with vital anxiety, the efficacy of NSD was considerably lower. Individuals with the duration of the phase under one month and the greatest reduction of the symptomatology after the recovery night showed the greatest response to the treatment. Favourable prognostic signs included such symptoms as psychomotor inhibition, lassitude and apathy, although the tempo of their reduction in the process of treatment considerably lagged behind improvement in the general mood, sleep and regress of autonomous disorders. Activation of the noradrenergic system is assumed to be one of the main mechanisms of NSD action.