The peripheral interactions of amphetamine with antipsychotic agents may elucidate some central mechanisms by which these drugs affect the behavioral responses to amphetamine. The authors studied the effects of intravenous amphetamine on plasma levels of norepinephrine (NE), dopamine-beta-hydroxylase (DBH), pulse rate, and blood pressure in schizophrenic patients. Amphetamine increased plasma NE, pulse rate, and blood pressure without significantly changing plasma DBH. DBH activity was similar in drug-free schizophrenic and normal subjects. Neither pimozide nor lithium altered these amphetamine effects nor changed any of the cardiovascular parameters measured in the drug-free subjects. Pimozide and lithium alter behavior and the behavioral effects of amphetamine, but neurotransmitters other than NE may be involved.