Antagonists of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor have been reported to potentiate the antiparkinsonian action of levodopa and reverse levodopa-induced motor fluctuations in animal models of Parkinson's disease. To evaluate the effect of NMDA receptor blockade on dyskinesias complicating the response to long-term levodopa therapy, we studied the selective antagonist LY235959 in six 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned monkeys. Drugs were administered subcutaneously, LY235959 at doses of 0.5, 1.0, 3.0, and 5.0 mg/kg and levodopa/benserazide at doses that produced moderate dyskinesias while almost totally reversing parkinsonian signs. Compared with vehicle control injections, LY235959 (3.0 mg/kg) abolished oral dyskinesias and diminished choreic dyskinesias by 68% (p < 0.01). Lower doses had smaller effects, although still significant, on oral dyskinesias (55% reduction at 1.0 mg/kg, p < 0.05). The highest LY235959 dose (5.0 mg/kg) prolonged oral dyskinesia suppression, but tended to increase dystonia severity. LY235959 had no effect on motor function when given alone and did not reduce the antiparkinsonian response to levodopa. These findings suggest that NMDA receptor blockade may ameliorate the dyskinetic complications of long-term levodopa therapy, without diminishing the beneficial effects on parkinsonian signs.