The autoradiographic distribution and density of muscarinic receptors was studied in the neostriatum of rats with long-term unilateral 6-hydroxydopamine (6-OHDA) lesions of the nigrostriatal dopaminergic pathway and in lesioned rats who had additionally received embryonic substantia nigra grafts in the dopamine denervated striatum. Muscarinic receptors were labeled with [3H]quinuclidinyl benzilate (QNB), M1 receptors were directly labeled with [3H]pirenzepine (PZ) and non-M1 receptors were labeled by the competition of 100 nM PZ with [3H]QNB. The density and distribution of muscarinic receptors were directly compared to the sodium-dependent, high-affinity, choline uptake sites as labeled with [3H]hemicholinium-3 (HC-3). In the 6-OHDA-lesioned animals, there was a 25% reduction in muscarinic receptors labeled with [3H]QNB. Subtype analysis showed that there was a reduction of both M1 (-26%) and non-M1 (-33%) receptors. A normal density of both muscarinic receptor populations was found in animals with successful transplants. Saturation analysis demonstrated that the changes, in muscarinic receptor density, were due to a change in receptor number (Bmax) and not affinity (Kd). There was no significant change in [3H]HC-3 binding in the 6-OHDA-lesioned or transplanted animals, indicating that alterations in muscarinic receptors were not due to transynaptic degeneration of striatal cholinergic interneurons. The findings of downregulation of muscarinic receptors following long-term dopamine denervation and the subsequent normalization of muscarinic receptor density after fetal mesencephalic transplantation suggests that transplanted substantia nigra cells are able to restore inhibitory control on striatal cholinergic interneurons.