This study explored the hypothesis that Gilles de la Tourette's syndrome involves a disturbance in arousal modulation. The experimental group consisted of 20 unmedicated subjects with the Gilles de la Tourette's syndrome, and the control group of 20 subjects with chronic medical illnesses (haemophilia, von Willebrandt's disease and diabetes). There were differences between groups in the manner in which log conductance level changed over time during sound and light habituation experiments involving mild levels of stimulation with the Gilles de la Tourette group showing less change in arousal level over trials than the control group. No group differences were found in measures of phasic arousal, rate of spontaneous fluctuations and performance on two tasks that have been related to cortical arousal. It is suggested that the slower change in log conductance level in the Gilles de la Tourette group during the sound and light habituation experiments indicates that reticular activity is more persistent in these patients.