SUMMARY Pig posterior pituitary lobe powder contains a peptide structurally identical to the 18–39 portion of porcine adrenocorticotrophin (ACTH). Its extraction by several different procedures is described and its susceptibility to degradation in the presence of 5% acetic acid has been noted. This degradation has been ascribed to the presence of acid proteases in the acetone-dried powder of the posterior pituitary lobe. The peptide has been isolated by chromatography on Biogel P6 and DEAE-cellulose, and characterized by enzyme fragmentation studies. It resembles the peptide isolated from rat neurointermediate lobes termed 'corticotrophin-like intermediate lobe peptide' and it is suggested that it is derived by the intracellular cleavage of ACTH in the pars intermedia cells, with subsequent formation of α-melanocyte-stimulating hormone from the other adrenocorticotrophic fragment.