The go-no go test requires a subject to emit a simple motor response to one cue while inhibiting the response in the presence of another cue. This test has been effective in demonstrating impulsivity (elevated commission error rate) in children with attention deficit disorder (ADD). In this study, we examined the effects on go-no go test performance of two doses of methylphenidate (0.15 mg/kg and 0.3 mg/kg) administered in double-blind placebo-controlled fashion to children with ADD. Our results indicate that even modest doses of methylphenidate improve the go-no go performance of these children by decreasing their tendency to make impulsive commission errors. Thus the test is sensitive to the effects of methylphenidate and can be used to monitor a response to therapy.