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A group of four monkeys was rendered parkinsonian with the toxin MPTP. They were then treated chronically with L-DOPA/benserazide 50/12.5 mg/kg given orally daily for 2 months. This dose produced a striking antipar-kinsonian effect, but all animals manifested dyskinesia. A series of agents acting primarily on neurotransmitters other than dopamine were then tested in combination with L-DOPA to see if the dyskinetic movements would be modified. Several drugs, including clonidine, physostigmine, methysergide, 5-MDOT, propranolol, and MK-801, markedly reduced the dyskinetic movements but at the cost of a return of parkinsonian symptomatology. However, yohimbine and meperidine reduced predominantly the dyskinetic movements. Baclofen was also useful in one monkey against a more dystonic form of dyskinesia. Atropine converted the dystonic movements into chorea.