The immunocytochemical distribution of [Leu]enkephalin and an adrenal enkephalin precursor fragment (BAM-22P) immunoreactivity was investigated in the diencephalon and brainstem of rats pretreated with relatively high doses of colchicine (300-400 micrograms/10 microliters intracerebroventricularly). The higher ranges of colchicine pretreatment allowed the visualization of extensive enkephalin-containing systems in these brain regions, some of which are reported for the first time. Immunoreactive perikarya were found in many hypothalamic and thalamic nuclei, interpeduncular nucleus, substantia nigra, the colliculi, periaqueductal gray, parabrachial nuclei, trigeminal motor and spinal nuclei, nucleus raphe magnus and other raphe nuclei, nucleus reticularis paragigantocellularis, vestibular nuclei, several noradrenergic cell groups, nucleus tractus solitarius, as well as in the spinal cord dorsal horn. In addition to the above regions, immunoreactive fibers were also noted in the habenular nuclei, trigeminal sensory nuclei, locus coeruleus, motor facial nucleus, cochlear nuclei, dorsal motor nucleus of the vagus, and hypoglossal nucleus. When adjacent sections of those stained for [Leu]enkephalin were processed for BAM-22P immunoreactivity, it was found that these two immunoreactivities were distributed identically at almost all anatomical locations. BAM-22P immunoreactivity was generally less pronounced and ws preferentially localized to neuronal perikarya. The results of the present as well as the preceding studies (Khachaturian et al., '83) strongly suggest substantial structural similarity between the adrenal proenkephalin precursor and that which occurs in the brain. Also discussed are some differences and parallels between the distribution of [Leu]enkephalin and dynorphin immunoreactivities.