Lipotropin and peptides related to β-endorphin were extracted from the anterior pituitary and the pars intermedia of porcine pituitary and were resolved by gel exclusion and ion exchange chromatography. Possible heterogeneity in the structure of the lipotropin was investigated by identifying the C-terminal fragment released by limited proteolysis with trypsin; the cleavage was restricted to the carboxyl group of arginine residues by employing citraconylation to protect the ε-NH2 groups of lysine. The lipotropin obained from both regions of the pituitary gave rise to the same C-terminal peptide which contained the 31-residue sequence of β-endorphin; none of the 26- and 27-residue forms was detected. In contrast, the β-endorphin-related peptides that were isolated directly from the pars intermedia exhibited a high degree of C-terminal proteolysis: they were present principally as the 26- and 27-residue peptides. The results demonstrate that lipotropin differs from β-endorphin in that it occurs exclusively in the form that contains the full C-terminal sequence. It is concluded that during biosynthesis lipotropin undergoes conversion to β-endorphin before proteolysis takes place at the C-terminus. The processing reactions that convert lipotropin to β-endorphin 1–31 and β-endorphin 1–31 to β-endorphin 1–27 are thus ordered and not competitive. The results also indicate that glycylglutamine, the bioactive C-terminal dipeptide of lipotropin, is formed from β-endorphin and not f