A stop signal paradigm in combination with heart rate measures were used to test the hypothesis of deficient inhibitory processing in boys with attention deficit hyperactivity disorder (ADHD). The authors measured the inhibitory process initiated by a stop signal by observing its success, its latency using a horse race model, and its impact on the timing of the heartbeat. Boys with ADHD performed well and showed appropriate psychophysiological changes. Inhibition latencies were longer in the boys with ADHD, particularly those with concurrent oppositional defiant disorder, than in the controls. Cardiac evidence suggested that greater preparation was associated with inhibition failures in controls, but not in the boys with ADHD. Boys with ADHD seem capable of attending carefully to control responses, but this control appears less effective and arguably is more effortful than in other boys.