We have recently demonstrated that [3 H] ohmefentanyl, a non-peptidergic opioid ligand which was suggested to cross the blood brain barrier in contrast to other peptidergic opioid ligands, bound not only to μ opioid receptor sites but also toσ sites. In order to examine whether [3 H] ohmefentanyl can be used as a marker for μ sites, we investigated the effects of brain lesions on [3 H] ohmefentanyl binding site densities, as compared with [3 H][D-Ala 2, MePhe 4, Gly-ol 5] enkephalin ([3 H] DAGO), a selecticve μ ligand. These binding site dinsities were measured by quantitative auto-rediography in the rat striatum and substantia nigra, two brian structures known to contain a high density of μ receptors, following lesions of the nigro-striatal dopaminergic pathway and striatal intrinsic neurons. Following unilateral nigral lesion with 6-hydroxydopamine,[3 H] ohemfentanyl binding site densities were decreased in the patches (-35%) and matrix (-20%) of the ipsilateral striatum and in the lisioned substantia nigra pars compacta (-49%). Unilateral striatal lesion with quinolinic acid induced 72%, 61% and 50% decreases in [3 H] ohmefentanyl binding in the patches and matrix of the lesined striatum and in the ipsilateral substantia nigra pars reticulate, respectively. Similar results were obtained in the binding of [3 H] DAGO. Indeed, a significant linear correlation was observed between [3 H]-ohmefentanyl and [3 H] DAGO binding site densities. Therefore, μ opioid receptors may be mainly located on intrinsic neurons in the striatum, dopaminergic cell bodies in the substantia nigra pars compacta and nerve terminals of striatal efferents in the substantia nigra pars reticulata. The present study revealed that [3 H] ohmefentanyl binding sites follow a pattern similar to that observed for μ opioid receptors in response to lesions of the nigro-striatal dopaminergic pathway and striatal intrinsic neurons. Possible binding of [3 H] ohmefentanyl to σ sties may not influence changes in μ sites caused by such lesions.[3 H] Ohemefentanyl may thus be a useful tool as a marker for μ opioid receptors.