Erythrocyte membrane Na,K-ATPase activity of manic patients was studied. The activity of patients in acute manic phase was higher than that of normal controls, while that of patients in normal phase was not. Even in the same patient, the activity in acute phase was higher than that in normal phase. Mg-ATPase activities did not differ between controls and patients. These findings clearly indicate a possible correlation between change in clinical phase in manic patients and change of erythrocyte membrane Na,K-ATPase activity.