Previous studies have demonstrated that the adrenocorticotropic hormone (ACTH) and its 4-10 fragment are potent modulators of attention in humans. In these studies the Nd component ('negative displacement') of the stimulus-evoked brain potential (event-related potential, ERP), which is a neurophysiological indicator of selective attention, proved most sensitive to effects of these peptides. The Nd decreased in amplitude linearly with logarithmically increasing doses of ACTH 4-10 applied. Here we report results complementing these previous findings by establishing a time course of the effects of ACTH 4-10 on human attention. In a double-blind cross-over study, attention performance on a dichotic listening task was tested in 18 healthy volunteers, after having received intravenously either placebo or 1 mg ACTH 4-10. Tests took place 0, 30, 90, 180, 330, 600 min, 24, 48 and 72 h after drug intake and took about 15 min. Each subject was tested on 4 of these occasions, selected according to a random schedule. During task performance ERPs to the different task stimuli were recorded. ERPs were used to determine the Nd (an indicator of controlled processing of stimuli which the subjects selectively attended to) and the mismatch negativity (MMN) reflecting automatic processing of stimulus deviance. ACTH 4-10 diminished the Nd significantly at 30 min after treatment administration. On none of the other occasions significant changes in Nd occurred. MMN was not affected by the peptide. The results indicate a short-term action of ACTH 4-10 on selective attention, which takes about 30 min to develop and does not last longer than 90 min after treatment.