We have examined the ability of KW-6002, an adenosine A2a antagonist, to modulate the dyskinetic effects of L-DOPA in 6-hydroxydopamine-lesioned rats. In animals rendered dyskinetic by a previous course of L-DOPA treatment, KW-6002 did not elicit any abnormal involuntary movements on its own, but failed to reduce the severity of dyskinesia when coadministered with L-DOPA. A second experiment was undertaken in order to study the effects of KW-6002 in L-DOPA-naive rats. Thirty-five animals were allotted to four groups to receive a 21-day treatment with: (i) KW-6002 (10 mg/kg/day); (ii) L-DOPA (6 mg/kg/day) i.p.; (iii) KW-6002 plus L-DOPA (same doses as above) or (iv) vehicle. Chronic treatment with KW-6002-only produced a significant relief of motor disability in the rotarod test in the absence of any abnormal involuntary movements. Combined treatment with L-DOPA and KW-6002 improved rotarod performance to a significantly higher degree than did each of the two drugs alone. However, this combined treatment induced dyskinesia to about the same degree as did L-DOPA alone. In situ hybridization histochemistry showed that KW-6002 treatment alone caused an approximately 20% reduction in the striatal levels of preproenkephalin mRNA, whereas neither the coadministration of KW-6002 and L-DOPA nor L-DOPA alone significantly altered the expression of this transcript in the dopamine-denervated striatum. Either alone or in combination with L-DOPA, KW-6002 did not have any modulatory effect on prodynorphin mRNA expression or FosB/DeltaFosB-like immunoreactivity in the dopamine-denervated striatum. These results show that monotreatment with an adenosine A2a receptor antagonist can relieve motor disability without inducing behavioural and cellular signs of dyskinesia in rats with 6-hydroxydopamine lesions. Cotreatment with KW-6002 and L-DOPA potentiates the therapeutic effect but not the dyskinesiogenic potential of the latter drug.