A group of manic-depressive and depressed patients and a group of normal volunteers were tested on two measures of stimulus intensity control. The effects of lithium carbonate administration on stimulus intensity control were studied in the patient group. On baseline testing, both the perceptual judgment and EEG average evoked response measures of stimulus intensity control differentiated between manic-depressive and depressed patients. Lithium carbonate administration was associated with a decrease in average evoked response augmentation in both patient groups. Stimulus intensity control may provide a bridging concept between conventional psychiatric categories and underlying neurophysiological processes.