The peptides that represent the major components with alpha-endorphin- and gamma-endorphin-like immunoreactivity in the rat neurointermediate lobe were purified to homogeneity and chemically characterized. Rat neurointermediate lobes were extracted by boiling and homogenization in acetic acid. Peptide purification was based on gel filtration, followed by two high-pressure liquid chromatography steps. Pools containing peptides with the size and immunochemical properties of alpha- and gamma-endorphins were resolved by reverse-phase high-pressure liquid chromatography into multiple immunoreactive components. The major forms were finally purified by paired-ion high-pressure liquid chromatography. The amino acid compositions of these peptides fitted the beta-endorphin sequences 1-16 and 1-17. Tryptic mapping, aminopeptidase M digestion, chromatographic characterization, and immunoreactivity to an antiserum recognizing the N alpha-acetylated terminus of endorphins showed that these peptides were indistinguishable from N alpha-acetyl-alpha-endorphin (N alpha-acetyl-beta-endorphin 1-16), and N alpha-acetyl-gamma-endorphin (N alpha-acetyl-beta-endorphin 1-17). The NH2-terminal residue of the peptides was identified by mass spectrometry as N alpha-acetyltyrosine, substantiating the identity of the peptides. The results demonstrate the existence of N alpha-acetylated alpha- and gamma-endorphin as endogenous peptides in the neurointermediate lobe of the rat pituitary gland. In view of their occurrence and biological properties they should be considered significant members of the pro-opiomelanocortin family.