This paper aims to examine the effects of methylphenidate (MPH) on integrated baseline and event-related psychophysiological measures in AD/HD. Thirty-four unmedicated AD/HD adolescents (11-17 years; 6 females) were first compared to 34 age- and sex-matched controls, and then re-tested at least 4 weeks after methylphenidate (MPH) medication. In each testing session, EDA was recorded simultaneously with EEG during a resting eyes open condition, and with ERPs during an auditory oddball task. Unmedicated AD/HD subjects were compared to controls and then AD/HD subjects were compared pre- and post-medication. Correlations between the change in EEG theta and the remaining psychophysiological variables were undertaken to provide information about post MPH treatment changes. In the unmedicated state, AD/HD was characterized by abnormally enhanced theta, across fronto-central sites, generally reduced P2 responses, with larger non-specific and oddball-elicited SCRs and poor behavioural performance on the oddball task. Following treatment, AD/HD showed a 'normalization' of theta activity (particularly in the right hemisphere), a reduction in the rate of decrement of EDA and a general increase in P3 amplitude. These findings suggest that methylphenidate is associated with a robust 'normalization' of low frequency EEG activity during the resting brain state, but has less impact on task-related brain activity or phasic changes in autonomic function. This dissociation of resting and task-related activity may prove to be useful in elucidating the effects of stimulant versus new non-stimulant medications in AD/HD.