In rodents, serotonin (5-HT) antagonists counteract behavioral and biochemical effects of neuroleptic drugs. Therefore, we have studied the effect of different 5-HT drugs and one anticholinergic drug in acute dystonia in five cebus monkeys chronically treated with haloperidol. Acute dystonia induced by subcutaneous injections of haloperidol was slightly reduced by the 5-HT antagonist methysergide (4.0 mg/kg), while mianserin, ketanserin, and ritanserin (R 55 667; a new selective and potent 5-HT receptor blocker) had no effect. This was contrasted by the marked antidystonic effect of the anticholinergic drug biperiden (0.05–1.0 mg/kg). The 5-HT agonist citalopram, a specific 5-HT uptake inhibitor, had no significant effect. It is concluded that 5-HT antagonists have no useful effect in neuroleptic-induced dystonia.