The distribution of mu opioid receptors was examined by light and electron microscopic autoradiography in the locus coeruleus of the rat following in vitro labelling with the iodinated agonist [125I]FK-33824. At the light microscopic level, specific mu opioid binding sites were concentrated over the perikarya and dendrites of neurons that were tyrosine hydroxylase-immunopositive in adjacent sections. Accordingly, both the number of tyrosine hydroxylase-immunoreactive neurons and the density of labelled mu receptors decreased markedly throughout the rostrocaudal extent of the nucleus following treatment with the catecholaminergic neurotoxin 6-hydroxydopamine. By electron microscopy, specifically labelled receptors were detected both inside and on the surface of locus coeruleus neurons. Intracellular sites were found by resolution circle analysis to be highly concentrated within the endoplasmic reticulum and Golgi apparatus, suggesting that the ligand recognizes both glycosylated and preglycosylated forms of receptor. The remainder were found mainly over the cytoplasmic matrix or intracytoplasmic vesicles, and were attributed to newly synthesized or recycled receptors in transit. Cell surface receptors were present over both dendritic and perikaryal membranes of noradrenergic cells. These were most highly concentrated opposite abutting axon terminals, suggesting the existence of receptor 'hot spots' at sites of putative endogenous ligand release. However, only a small proportion of these sites was associated with synaptic specializations. Furthermore, an important contingent was detected opposite non-axonal elements, such as dendrites and glial cells, suggesting that mu opioid ligands act mainly parasynaptically on locus coeruleus neurons. Finally, approximately 5% of labelled receptors were associated with axoglial interfaces, indicating that a minor action of mu opioids in the locus may be presynaptic and/or glial.