mu-Opioid receptor, the opioid receptor that shows the highest affinity for morphine, appears to induce a variety of side-effects, at least partly, directly through the mu-opioid receptor on neurons constituting the autonomic part of the vagus nerve. Thus, in the present study, location of mu-opioid receptor-like immunoreactivity in the central components of the autonomic part of the vagus nerve was examined in the rat. The intense immunoreactivity was observed light microscopically in the neuropil of the commissural subnucleus and the dorsal part of the medial subnucleus of the nucleus of the solitary tract, and in the neuropil of the rostral half of the ambiguus nucleus. The immunoreactivity was moderate in the neuropil of the rostral and lateral subnuclei and ventral part of the medial subnucleus of the nucleus of the solitary tract, and weak in the neuropil of the dorsal motor nucleus of the vagus nerve. In the nodose ganglion, many neurons of various sizes (17-48 microns in soma diameter) showed moderate immunoreactivity. After unilateral vagotomy at a level proximal to the nodose ganglion, the immunoreactivity in the ipsilateral ambiguus nucleus was apparently reduced within 48 h of the operation, and completely disappeared by the seventh day after the operation. In the nucleus of the solitary tract and dorsal motor nucleus of the vagus nerve, the reduction of immunoreactivity after the ganglionectomy was detectable on the fourth day after the operation, and became readily apparent by the seventh day after the operation; the immunoreactivity, none the less, still remained on the 10th day after the operation. Electron microscopically, the immunoreactivity in the ambiguus nucleus was seen mainly on dendritic profiles and additionally on somatic ones; no immunoreactivity was detected in axonal profiles. The immunoreactivity in the dorsal motor nucleus of the vagus nerve was observed only on dendritic profiles. The immunoreactivity in the nucleus of the solitary tract was seen on axonal and dendritic profiles, but not on somatic profiles. The immunoreactive axon terminals in the nucleus of the solitary tract were filled with spherical synaptic vesicles and made asymmetric synapses with dendritic profiles. The results indicate that the mu-opioid receptor in the central components of the autonomic part of the vagus nerve is located on dendrites and cell bodies of efferent neurons in the ambiguus, on dendrites of efferent neurons in the dorsal motor nucleus, and on axons which arise from nodose ganglion neurons and terminate in the nucleus of the solitary tract. The receptors on these structures may constitute the targets of enkephalin-containing and beta-endorphin-containing afferent axons arising from brainstem neurons. The receptors on the axon terminals of nodose ganglion neurons may be involved in regulation of the release of neurotransmitters and/or neuromodulators.