Before treatment 49 obsessive-compulsive (o-c) ritualizers were presented with two series of brief stimuli – 15, 100db tones (brief neutral) and 15 presentations of a ritual-evoking object (brief aversive). Habituation of skin conductance (SC) responses to the tones was reduced compared with that previously found in normal subjects. Neither habituation rates to tones nor aversive stimuli were related to coexisting anxiety or depression or to the severity of o-c symptoms. The increased arousal induced by the aversive stimuli was sustained, that induced by the tones was short-lived and SC level and subjective anxiety had returned to resting levels by the end of the tone series. Concordance between SC activity and subjective anxiety was much greater during and after the presentations of ritual evoking stimuli than of tones. There were few correlations between SC and clinical measures, though patients who strongly resisted and were able to control their compulsive urges were more aroused.