l-DOPA-induced dyskinesia (LID) is a major complication of the pharmacotherapy of Parkinson's disease. Emerging approaches to the treatment of LID include negative modulation of metabotropic glutamate receptor type 5 (mGluR5) and positive modulation of serotonin receptors 5-HT1A/1B. We set out to compare the efficacy of these two approaches in alleviating the dyskinesias induced by either l-DOPA or a D1 receptor agonist. Rats with unilateral 6-OHDA lesions were treated chronically with either l-DOPA or the selective D1-class receptor agonist SKF38393 to induce abnormal involuntary movements (AIMs). Rats with stable AIM scores received challenge doses of the mGluR5 antagonist, MTEP (2.5 and 5 mg/kg), or the 5-HT1A/1B agonists 8-OH-DPAT/CP94253 (0.035/0.75 and 0.05/1.0 mg/kg). Treatments were given either alone or in combination. In agreement with previous studies, 5 mg/kg MTEP and 0.05/1.0 mg/kg 8-OH-DPAT/CP94253 significantly reduced l-DOPA-induced AIM scores. The two treatments in combination achieved a significantly greater effect than each treatment alone. Moreover, a significant attenuation of l-DOPA-induced AIM scores was achieved when combining doses of MTEP (2.5 mg/kg) and 8-OH-DPAT/CP94253 (0.035/0.75 mg/kg) that did not have a significant effect if given alone. SKF38393-induced AIM scores were reduced by MTEP at both doses tested, but not by 8-OH-DPAT/CP94253. The differential efficacy of MTEP and 8-OH-DPAT/CP94253 in reducing l-DOPA- versus SKF38393-induced dyskinesia indicates that these treatments have different mechanisms of action. This contention is supported by the efficacy of subthreshold doses of these compounds in reducing l-DOPA-induced AIMs. Combining negative modulators of mGluR5 with positive modulators of 5-HT1A/1B receptors may therefore achieve greater than additive antidyskinetic effects and reduce the dose requirement for these drugs in Parkinson's disease.