Abstract.Specific activities of erythrocyte membrane Na‐K ATP‐ase, Mg ATP‐ase and Ca‐Mg ATP‐ase were measured in twenty‐five patients with endogenous depression both during acute phase of the illness and in remission as well as in sixteen healthy subjects. The three ATP‐ase activities were significantly lower during depressive phase than in remission period. The ATP‐ase values of depressive patients in remission did not differ from those of healthy controls. No difference was found in respect to parameters studied between the group of patients with depression in the course of bipolar manic‐depressive psychosis and patients with unipolar depression both in depression and remission. These data may suggest that the reduced activity of ATP‐ases in depressed patients is not due to a primary genetic defect but is an episodic change associated with depression. In view of consistency of this phenomenon its diagnostic and research potential is wort