In order to assess the possibility of altered serotonergic responsivity in antisocial personality disorder with substance abuse (ASP), 15 men with ASP and 12 controls were challenged with the serotonin agonist, m-chlorophenylpiperazine (m-CPP), and prolactin and cortisol responses were evaluated. Psychometric measures of hostility and aggression, impulsivity, cognitive tempo, and various aspects of sociopathy were also obtained. ASP subjects had a significantly reduced prolactin response to m-CPP compared with controls, and a significantly greater cortisol response. The prolactin responses showed a significant inverse correlation with measures of assaultive aggression, hypophoria (negative affects), and increased needs. There was no significant correlation found between cortisol responses and any of the psychometric measures. Impulsivity as characterized either by behavioral self-report or measurement of cognitive tempo did not correlate with either prolactin or cortisol responses. A discriminant function analysis depicted ASP subjects as displaying resentment towards others and having poor test-taking efficiency, heightened irritability, and diminished prolactin response to m-CPP. Using these four criteria, nearly 93% of subjects were successfully classified. These results suggest that altered serotonergic function is associated with assaultiveness and dysphoria but not impulsivity in individuals with ASP.