The non competitive antagonist of N-methyl-D-aspartate (NMDA) receptors MK-801, at doses which did not produce any behavioural modification per se, increased by about 40% the contralateral turning induced by the dopaminergic (DA) D-1/D-2 agonist L-dopa in rats bearing a unilateral lesion of the DA nigro striatal neurons. Administration of MK-801 in combination with selective agonists of the D-1 (SKF 38393) or D-2 (LY 171555) receptor, induced an increase of about 280% of D-1 mediated turning and a 70% decrease of D-2 mediated turning. Analysis of the potentiation of L-dopa mediated turning by MK-801 after selective D-1 receptor blockade by SCH 23390, revealed that the increase of turning was related to the stimulation of D-1 receptors by L-dopa. Immunocytochemical visualization of the proto-oncogene c-fos in the caudate-putamen (CPu) of lesioned rats, revealed a sparse c-fos positive nuclei in the lesioned CPu of rats treated with SKF 38393 alone, while after combined administration of MK-801 and SKF 38393 dense labelling of nuclei was found. The results indicate that blockade of NMDA receptors by MK-801 acts synergistically with D-1 agonists in the induction of turning after DA denervation and shows that these changes are correlated with c-fos induction in specific areas of the CPu.