The distribution of five major products of proenkephalin B [dynorphin1-17, dynorphin B, dynorphin1-8, alpha-neo-endorphin and beta-neo-endorphin] was studied in regions of rat brain and pituitary. The distribution pattern of immunoreactive (ir) dynorphin B (= rimorphin) was found to be similar to that of ir-dynorphin1-17, with the highest concentrations being present in the posterior pituitary and the hypothalamus. HPLC and gel filtration showed the tridecapeptide dynorphin B to be the predominant immunoreactive species recognized by dynorphin B antibodies in all brain areas and in the posterior pituitary. In addition, two putative common precursor forms of dynorphin B and dynorphin1-17 with apparent molecular weights of 3,200 and 6,000 were detected in brain and the posterior pituitary. The 3,200 dalton species coeluted with dynorphin1-32 on HPLC. In contrast with all other tissues, anterior pituitary ir-dynorphin B and ir-dynorphin1-17 consisted exclusively of the 6,000 dalton species. Concentrations of dynorphin1-8 were several times higher than those of dynorphin1-17 in striatum, thalamus, and midbrain while posterior pituitary, hypothalamus, pons/medulla, and cortex contained roughly equal concentrations of these two opioid peptides. No dynorphin1-8 was detected in the anterior pituitary. Concentrations of beta-neo-endorphin were similar to those of alpha-neo-endorphin in the posterior pituitary. In contrast, in all brain tissues alpha-neo-endorphin was found to be the predominant peptide, with tissue levels in striatum and thalamus almost 20 times higher than those of beta-neo-endorphin. These findings indicate that differential proteolytic processing of proenkephalin B occurs within different regions of brain and pituitary. Moreover, evidence is provided that, in addition to the paired basic amino acids -Lys-Arg- as the "typical" cleavage site for peptide hormone precursors, other cleavage signals also seem to exist for the processing of proenkephalin B.