Dopaminoceptive system alterations in the basal ganglia have been implicated in the pathogenesis of wearing-off fluctuations that complicate levodopa therapy of Parkinson's disease. To evaluate the contribution of glutamatergic mechanisms to the associated changes in striatal efferent pathway function, we examined the ability of N-methyl-D-aspartate (NMDA) receptor blockade to modify the motor response changes produced by chronic levodopa administration to hemiparkinsonian rats. Unilaterally 6-hydroxydopamine lesioned rats, given levodopa/benserazide (25/6.25 mg/kg) twice daily for 3 weeks, developed a progressive shortening in the duration of their motor response to levodopa similar to that occurring in parkinsonian patients with wearing-off phenomenon. The acute systemic administration of MK-801 (0.1 mg/kg) to these animals completely reversed the decrease in turning duration (P < 0.01). Intrastriatal injection of the NMDA antagonist was even more effective in prolonging the levodopa response (P < 0.01), while intranigrally injected MK-801 produced no statistically significant change in the duration of levodopa-induced rotation. Rotational intensity was unaffected by all routes of MK-801 administration. These results suggest that drugs capable of blocking NMDA receptors, especially in striatum, may help ameliorate motor fluctuations in patients with advanced Parkinson's disease.