The authors used a combined behavioral and neuroendocrinological strategy to investigate the relevance of abnormalities in the brain dopaminergic systems to the pathophysiology of tardive dyskinesia by assessing the effects of apomorphine, a directly acting dopamine agonist, and d-amphetamine, an indirectly acting dopamine agonist, in patients with tardive dyskinesia. Administration of I.V. d-amphetamine increased dyskinetic movements in most patients with tardive dyskinesia, a finding consistent with the dopaminergic hypothesis. Contrary to predictions based on animal models, apomorphine did not increase dyskinetic movements in these patients but instead substantially reduced dyskinesia in some patients. Patients with tardive dyskinesia did not have a greater drop in serum prolactin or a greater rise in serum growth hormone after apomorphine than normal or chronic schizophrenic subjects without tardive dyskinesia.