Selective D1 and D2 dopamine (DA) antagonists and agonists were given to 4 Cebus monkeys who had previously received haloperidol treatment for 4 years. SCH 23390 (a selective D1 antagonist) and raclopride (a selective D2 antagonist) induced identical syndromes consisting of dystonia and oral dyskinesia. Biperiden (an anticholinergic drug) and LY 171555 (a selective D2 agonist) completely antagonized the dystonia and dyskinesia induced by SCH 23390 as well as raclopride. The combined treatment with LY 171555 and SCH 23390 (but not LY 171555 and raclopride) caused pronounced sedation. LY 171555 induced repetitive movements of head, legs and trunk, but no oral dyskinesia. SKF 38393 (a partial D1 agonist) caused slight sedation, minimal oral dyskinesia and a significant reduction in D2 agonist-induced repetitive movements.