The prohormone of beta-endorphin is unusual in that it is the precursor of more than one biologically active peptide (Fig. 1). The activation of this prohormone, to produce corticotropin (ACTH), alpha-melanotropin (alpha-MSH) and beta-endorphin, would seem to be relatively complex as its processing pattern is known to differ between tissues. Thus ACTH is produced in the anterior pituitary whereas alpha-MSH is formed in the pars intermedia; similarly, lipotropin and beta-endorphin seem to predominate in the anterior pituitary whereas beta-endorphin alone has been thought to be the principal component in the pars intermedia. We report here a study of the distribution of beta-endorphin-related peptides in various regions of porcine pituitary. The main products in the anterior pituitary were lipotropin and the potent analgesic form of beta-endorphin, whereas the main products in the pars intermedia were the inactive lipotropin C'-fragment and its N-acetyl derivative. Thus the processing of the C-terminal region of the beta-endorphin prohormone differs markedly between the two regions of porcine pituitary.