Three opioid receptor types have recently been cloned that correspond to the pharmacologically defined mu, delta and kappa 1 receptors. In situ hybridization studies suggest that the opioid receptor mRNAs that encode these receptors have distinct distributions in the central nervous system that correlate well with their known functions. In the present study polyclonal antibodies were generated to the C terminal 63 amino acids of the cloned mu receptor (335-398) to examine the distribution of the mu receptor-like protein with immunohistochemical techniques. mu receptor-like immunoreactivity is widely distributed in the rat central nervous system with immunoreactive fibers and/or perikarya in such regions as the neocortex, the striatal patches and subcallosal streak, nucleus accumbens, lateral and medial septum, endopiriform nucleus, globus pallidus and ventral pallidum, amygdala, hippocampus, presubiculum, thalamic and hypothalamic nuclei, superior and inferior colliculi, central grey, substantia nigra, ventral tegmental area, interpeduncular nucleus, medial terminal nucleus of the accessory optic tract, raphe nuclei, nucleus of the solitary tract, spinal trigeminal nucleus, dorsal motor nucleus of vagus, the spinal cord and dorsal root ganglia. In addition, two major neuronal pathways, the fasciculus retroflexus and the stria terminalis, exhibit densely stained axonal fibers. While this distribution is in excellent agreement with the known mu receptor binding localization, a few regions, such as neocortex and cingulate cortex, basolateral amygdala, medial geniculate nucleus and the medial preoptic area fail to show a good correspondence. Several explanations are provided to interpret these results, and the anatomical and functional implications of these findings are discussed.