L-3,4-Dihydroxyphenylalanine (L-DOPA) is the most effective treatment for Parkinson's disease, but chronic administration is complicated by the development of dyskinesia. We have previously demonstrated that the dopamine D4 receptor antagonist L-745,870 reduces the severity of L-DOPA-induced dyskinesia in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned macaque without compromising L-DOPA antiparkinsonian benefits. In the current study, we have addressed the effects of L-745,870 on the expression of L-DOPA-induced abnormal involuntary movements (AIMs) in the 6-hydroxydopamine-lesioned rat. Rats were primed with repeated L-DOPA administration, after which acute challenges of L-DOPA/L-745,870 (vehicle, 0.1, 0.3 and 1 mg/kg) were administered, and AIMs were assessed. Rotarod performance and AIMs were assessed. In L-DOPA-primed rats, L-745,870 (1 mg/kg, but not lower doses) alleviated previously established AIMs (by 84%, P<0.001). Whereas rotarod performance was significantly improved by L-DOPA/vehicle treatment, L-DOPA/L-745,870 failed to improve rotarod performance (P>0.05), suggesting that, in contrast to the MPTP-lesioned macaque, L-745,870 reduces L-DOPA antiparkinsonian benefit in the rat model. Overall, these data suggest that L-745,870 may have a narrow therapeutic window as an antidyskinetic agent in advanced Parkinson's disease.