The nature of impulsivity in hyperactive and conduct-disordered children was examined in two experiments, one involving a priming task, the other a delayed reaction time task. Four groups of children, aged 7 to 8 years and with IQs in the normal range, were recruited for study: (1) a pure hyperactive group (HA), (2) a hyperactive/conduct-disordered group (HA+CD), (3) a pure conduct-disordered group (CD), and (4) a normal control group (N). When the stimulus configuration and presentation were simple and well organized, none of the three clinical groups displayed any sign of impulsivity at the input/perceptual stage; there was no tendency to rush responding before adequate consideration of the relevant stimuli, i.e. a trading of accuracy for speed. Instead, the HA children were found to be disinhibited at the output/motor stage, i.e. failing to temporarily withhold activated responses. This deficit was found to be specific to the HA children; it was not observed in the CD and HA+CD children.