Endomorphin-1 and -2 are potent and selective agonists for the mu-opioid receptor which have recently been isolated from bovine brain extracts. In the present study we used polyclonal antibodies specific for endomorphin-2 to determine its immunocytochemical distribution in the rat brain and spinal cord Endomorphin-2-like immunoreactivity was confined to varicose fibers with an overall discrete distribution within the central nervous system. The immunostaining was completely abolished by preincubation of the antiserum with endomorphin-2 but not with endomorphin-1. Endomorphin-2-like immunoreactivity was enriched in many but not all brain regions known to contain dense mu-opioid receptors, including nucleus accumbens, septum, midline thalamic nuclei, hypothalamic and amygdala nuclei, locus coeruleus, periaqueductal gray and spinal cord dorsal horn. In contrast, endomorphin-2 was absent from the cortex, striatum, hippocampus, nucleus of the solitary tract and dorsal root ganglia. Dual-labeling experiments revealed that endomorphin-2-immunoreactive nerve fibers did not co-contain any other opioid peptide. Thus, the present findings strongly suggest that endomorphin-2 may be a natural ligand for the mu-opioid receptor likely to be involved in the modulation of nociceptive transmission and reward-seeking behavior.