Previous research on the behavioural functions of endogenous opioid systems in rodents suggested a possible opioid role in the regulation of attention. This proposal was consistent with reports that opiate administration in man impairs the ability to concentrate while opiate antagonists augment behavioural and electrophysiological indices of arousal and attention. We examined the effects of the opiate antagonist naloxone on electrophysiological measures of attention in normal human subjects, using a paradigm which dissociates selective information processing from concurrent processes of general arousal or alertness that may be present. We now report electrophysiological evidence that naloxone improves the selectivity of auditory attention in the presence of competing sources of stimuli. These findings indicate a role for the endogenous opioid systems in the regulation of selective attention in man.