Slow potential responses of 14 schizophrenic, 12 manic, and 14 depressed patients were studied when they were exhibiting acute symptoms and were medication free. Ten normal adults served as a control group. Statistically significant differences were demonstrated between the patients and normals. In the latter, a typical CNV appeared with warning-imperative stimulus contingencies. There was very little discrimination between resting and response activity in manic and depressed patients. Waveforms in the schizophrenics were more variable with negative potential changes after motor response. No other clinical correlates with direct current potential characteristics were identified. Test retest profiles were generally stable in all subjects, with some exceptions associated with somatic treatment.