Methylphenidate hydrochloride dextroamphetamine sulfate, and levamfetamine succinate have potential as pharmacologic tools for the indirect evaluation of the role of neurotransmitters in schizophrenia. In actively ill schizophrenic patients, methylphenidate administered intravenously causes a brief but clear intensification of preexisting psychotic symptoms, such as hallucinations and delusions. In our study, methylphenidate, dextroamphetamine, and levamfetamine were administered in equimolar doses to schizophrenic patients. Methylphenidate was a more effective activator of symptoms than dextroamphetamine, which in turn was more effective than levamfetamine. Levodopa (L-dopa) given orally also reportedly produces a temporary worsening of schizophrenic symptoms. While these findings augment a body of information suggesting that dopamine and norepinephrine may play a role in the activation of schizophrenic symptoms, our findings with methylphenidate (reportedly weak in eliciting stereotyped behaviour in rat) and our review of the literature indicate complexities that remain to be resolved. There is some utility of the procedure for differential diagnosis and selective therapy, but this is still of occasional and limited potential.