The concentration of immunoreactive (ir) beta-endorphin (beta-END) in the neurointermediate pituitary lobe was 15-fold higher in adult than in newborn rats; in contrast, that of ir-beta-END in the anterior lobe was twice as high in newborn as in adult animals. Ir-beta-END in the neurointermediate lobe of newborn rats consisted exclusively of beta-END-sized peptides, indicating that at birth rats are capable of processing the opioid peptide precursor proopiomelanocortin (POMC) to beta-END. Moreover, beta-END-related peptides in the neurointermediate lobe of newborn rats were found to be predominantly alpha-N-acetylated and, therefore, inactivated with respect to their opiate-like properties. Further analysis of these alpha-N-acetylated forms on high performance liquid chromatography indicated that newborn rats predominantly contained alpha-N-acetyl-(Ac-)beta-END-(1-31), whereas the major forms in adult rats were Ac-beta-END-(1-27) and -(1-26). Thus, the C-terminal processing of Ac-beta-END-(1-31) to -(1-27) and -(1-26) may not yet be fully active at birth, in contrast to the processing of POMC to beta-END. In the anterior lobe of newborn rats, however, the ratio of beta-lipotropin/beta-END resembled that of adults, and more than 80% of beta-END-sized ir-material was found to consist of nonacetylated (and therefore opiate-active) beta-END-(1-31), as in adults, suggesting that the enzymatic system responsible for processing of POMC to beta-lipotropin and beta-END is already mature at birth. The high concentrations of beta-END in the anterior lobe of newborn rats suggest a possible role of this opioid peptide in perinatal development and/or parturition.