Most of the early studies on the immunohistochemical distribution of enkephalin pentapeptide-like immunoreactivity used antisera that stained both proenkephalin- and prodynorphin-containing neurons. The present study used the peroxidase-antiperoxidase method, thick Vibratome sections and antisera specific for the carboxyl termini of [Met]enkephalin, [Met]enkephalyl-Arg6-Phe7, [Met]enkephalyl-Arg6-Gly7-Leu8, and metorphamide and for BAM 22P in order to obtain a detailed description of the distribution of authentic proenkephalin-containing perikarya and nerve processes. The peroxidase-antiperoxidase reaction product was intensified by the selective deposition of silver crystals in order to display the morphology of proenkephalin-containing neurons with great fidelity. The results indicate that the magnocellular perikarya in the supraoptic and paraventricular nuclei contain prodynorphin rather than proenkephalin as had been suggested by earlier investigators. The coarse fibers in the internal zone of the median eminence and the granule cell-mossy fiber pathway in the hippocampus also contain prodynorphin rather than proenkephalin. The number of proenkephalin-containing perikarya and/or the density of proenkephalin-containing nerve terminals in several other areas of the brain, e.g. the substantia nigra, the central amygdaloid nucleus, the periaqueductal gray and the parabrachial nuclei, were overestimated by earlier investigators. The distribution of authentic proenkephalin-containing perikarya and nerve processes is, despite these errors, similar to the distribution of enkephalin pentapeptide-like immunoreactivity described by earlier investigators. Proenkephalin-containing perikarya were identified for the first time in the medial and lateral habenular nuclei of the adult rat. Antisera specific for [Met]enkephalin, [Met]enkephalyl-Arg6-Phe7, [Met]enkephalyl-Arg6-Gly7-Leu8 and BAM 22P stain perikarya and nerve terminals with a similar distribution. The metorphamide antiserum also stains the same perikarya and nerve terminals; however, it also stains magnocellular perikarya in the zona incerta and the lateral hypothalamus that are not stained by any of the other proenkephalin-specific antisera.