To further elucidate the previously demonstrated protective actions of nicotine on lesioned nigrostriatal dopamine (DA) systems (Janson and Møller, Neuroscience, 57 (1993) 931-941), the present receptor binding experiments were carried out. Rats were partially hemitransected at the meso-diencephalic junction and the effects of chronic continuous (-)nicotine treatment (osmotic pumps s.c., 0.125 mg/kg/h, 14 days) on [3H]N-propylnorapomorphine ([3H]NPA) and [3H]methylcarbamylcholine ([3H]MCC) binding were investigated in striatal coronal sections to study the agonist binding sites of DA D2 receptors and nicotinic cholinoceptors, respectively. In saline-treated but not in nicotine-treated rats, the lesion led to an increased Bmax value of [3H]NPA binding. The Bmax value of [3H]MCC binding was increased by nicotine treatment and decreased by the partial hemitransection. These results indicate that chronic nicotine treatment counteracts the lesion-induced upregulation of the high-affinity agonist binding site of the DA D2 receptor, which may be explained by an increased presence of DA via a protective effect of nicotine on neostriatal DA terminals. This action of nicotine may be of interest in the treatment of neurodegenerative diseases such as Parkinson's disease.