The effect of methylphenidate preceded by a moderate dose of haloperidol on reaction times over the duration of a continuous performance test (CPT), was investigated in ten male children, with a DSM-III diagnosis of attention deficit disorder with hyperactivity disorder (ADDH). Using a within-subject double-blind design, the effects of methylphenidate preceded by haloperidol on reaction time during the first and second blocks of CPT test were compared. Methylphenidate maintained a significantly improved reaction time in the second block of the CPT test. When methylphenidate, preceded by placebo, was preceded by haloperidol this effect was not observed, suggesting opposing effects on attentional systems by methylphenidate versus haloperidol. The study is the first to examine the 'blocking' effect of haloperidol over the course of a CPT. The results suggest that dopamine systems are involved in the maintenance of the CPT response, and support an 'incentive motivation' theory of sustained attention.