Male, monozygotic twins (six pairs) were repeatedly tested before and after d-amphetamine, l-amphetamine, or placebo administration. Drug effects on cognitive, psychomotor, personality, mood, and pain variables were assessed. Members of a twin pair tended to respond similarly on several tests under placebo conditions, indicating genetic determination of the behavioral variables. In addition, cotwins tended to show similar responses to amphetamine as measured by one test of cognitive function, by several mood and personality variables (hostility, autonomic arousal, friendliness, feelings of tension and loss of control), and tended to have similar plasma levels of both amphetamine isomers. Although shared environmental effects cannot be ruled out, the results are consistent with genetic mediation of a variety of behavioral effects of amphetamines.