The precise distribution of the dopamine type D2 receptor has been mapped for the first time in rat brain using an antibody to D2 receptor protein. Polyclonal antisera were collected from rabbits inoculated with an undecapeptide identical to residues 24-34 of the D2 protein sequence. Rat brain slices, 40 microns in thickness, were incubated with either primary antiserum, the antiserum plus free peptide antigen, or pre-immune serum. Antibody binding was visualized by peroxidase-antiperoxidase (PAP) reaction followed by light microscopy. PAP complex bound moderately-to-densely throughout the medial forebrain bundle, and was seen in more discrete regions in the midbrain, consistent with the binding of D2 radioligands. There were some unexpected results, namely in the cerebral cortex and nucleus accumbens, there were unexpectedly steep gradients in binding density, decreasing caudally; no binding was detected in the hippocampus or the substantia nigra pars reticulata. In all positive-staining regions examined, the antibody was highly localized to neuronal cell bodies, except in the frontal cortex where antibody was also evident on basilar dendrites. These data confirm that the polyclonal antibody recognized dopamine D2 receptor protein throughout the rat brain, and suggest that the D2 receptor is distributed more abundantly on somata than on cellular processes.