Tardive dyskinesia is becoming detectable in increasing numbers of chronic psychiatric patients. More than 24 such patients were identified among approximately 400 ambulatory long-term patients at Boston State Hospital. This study demonstrates that tetrabenazine (Nitoman [Great Britain]), a dopamine-depleting agent, significantly reduces the frequency of abnormal mouth, jaw, and tongue movements at dosages up to 150 mg/day. The intensity of these movements and of associated abnormal movements of the extremities and trunk were also reduced. Reversible extrapyramidal symptoms were not increased at the dosage used. The antidyskinetic effect was marked in two thirds of the patients. The results of the study are consistent with the hypothesis that the abnormal movements of tardive dyskinesia might result from overactivity of dopaminergic neurones in the basal ganglia.