Peptidylglycine α-amidating activities from rat pituitary, brain and small intestine were compared, utilizing C-terminal analogues of vasoactive intestinal polypeptide (VIP), D-Tyr-Leu-Asn-Gly and D-Tyr-Asn-Gly, and C-terminal analogue of α-MSH, D-Tyr-Val-Gly. The three tissues had enzymic activities capable of converting the glycine-extended peptides to the corresponding α-amidated ones. In other words, all of three peptides could serve as substrates for the enzymes from both neural and gastrointestinal tissues. The activities were stimulated in the presence of copper and ascorbate; the optimal concentration of each cofactor was roughly equal for the three enzymes; similar pH profiles (a neutral pH optimum at 6.5-7 and another one at 8-8.5) were also observed. Desamide VIP-Gly was proved to be a potent inhibitor of the α-amidating activities from the tissues, but VIP was not, indicating that the α-amidating enzymes from these tissues in common have a recognition site for the C-terminal glycine of the glycine-extended precursor regardless of the length and nature of the sequence. No fundamental differences were observed between the catalytic properties of the alpha-amidating activities from these three tissues, raising the possibility that similar enzymes, which may or may not be a single species, are functioning in tissues that produce alpha-amidated polypeptides in vivo.