• Skin conductance, heart rate, and resting EEG were examined in 26 patients with unipolar and 24 with bipolar affective disorder (in remission) and 46 medical control patients. The electrodermal activity (EDA) was monitored during a tone series that included a dishabituating stimulus as well as at rest and during respiratory maneuvers. The EDA of the patients with affective disorder was uniformly depressed across all tasks and conditions. Only one of the unipolar and five of the bipolar patients showed an elevated tonic conductance level, compared with about half of the normal subjects. These results suggest that diminished EDA may serve as a marker of susceptibility to affective disorder or, alternatively, that elevated EDA may be a protective factor. There were no EEG differences between groups, and observed cardiac rate differences were attributable to the effects of tricyclic medication.