The postnatal development of several pro-enkephalin-B-derived opioid peptides – dynorphin1-17, dynor-phin1-8, dynorphin B, α-neo-endorphin and β-neo-endorphin – was examined in rat pituitary lobes. The concentrations of pro-enkephalin-B-derived peptides from the anterior pituitary were between 4- and 12-fold and those from the neurointer mediate pituitary between 17- and 122-fold lower in newborn as compared to adult rats. Similarly, the concentrations of vasopressin in the neurointermediate pituitary increased 50-fold between birth and adulthood; those of oxytocin, however, increased more than 540-fold over this period. The molecular weight pattern of dynorphin1-17, dynorphin1-8, dynorphin B, α- and β-neo-endorphin-immunoreactive peptides in the anterior and neurointermediate pituitary did not differ between 3-day-old pups and adult rats. In the neurointermediate pituitary, the major immunoreactive components had the same chromatographic properties as synthetic dynorphi1-17, dynorphin1-8, dynorphin B, α- and β-neo-endorphin, respectively, on gel filtration and high-performance liquid chromatography (HPLC). This indicates that neonatal rats were already capable of processing the precursor pro-enkephalin B into these various opioid peptides. In newborn rats, however, the amount of dynorphin1-8 in the neurointermediate pituitary was three times lower than that of its putative intermediate precursor peptide dynorphin1-17. Similarly, the amount of β-neo-endorphin was almost four times lower than that of its putative precursor α-neo-endorphin. In contrast, in the neurointermediate pituitary of adult rats, dynorphin1-17 and dynor-phin1-8 in addition to a α- and β-neo-endorphin, occurred in equimolar amounts. These findings suggest that the enzymatic processing of pro-enkephalin B to dynorphini_8 and β-neo-endorphin is carried out to a lesser extent in neonatal as compared to adult rats.