In vitro autoradiographic techniques were coupled with selective chemical lesions of the A10 dopamine cells and intrinsic perikarya of the A10 region to delineate anatomical localization of the mu-opioid receptors, labeled with 125I-Tyr-D-Ala-NMePhe-Gly-OH (125I-DAGO), and neurotensin receptors, labeled with 125I-[Tyr3]neurotensin within the mesolimbic dopamine system. Unilateral lesions of dopamine perikarya produced by 6-hydroxydopamine (6-OHDA), administered in the ventral mesencephalon, produced a unilateral loss of specific neurotensin binding (65%), but did not affect mu-opioid receptor density or distribution. Unilateral lesions of intrinsic perikarya by quinolinic acid (250 nmol) injected into the A10 dopamine region produced a significant reduction in mu-opioid receptors (50%), as well as a concomitant reduction in neurotensin receptors. Unilateral 6-OHDA- or quinolinic-induced lesions in the ventral mesencephalon failed to cause significant reduction in mu-opioid receptors in the caudate putamen or limbic forebrain. In contrast, mesencephalic lesions produced significant reductions (50%) in neurotensin binding in the caudate putamen and lateral nucleus accumbens. However, neurotensin binding within the medial nucleus accumbens and adjacent limbic nuclei were unaffected by these treatments. These results are consistent with the pharmacological effects of mu-opioids and neurotensin, and suggest an indirect modulation of the mesolimbic dopamine neurons by mu-opioid agonists and the endogenous opioid peptides.