Proteolytic digestion of gelatin extracts from Alaska Pollack (Theragra chalcogramma) skin brings about a high angiotensin I converting enzyme (ACE) inhibitory activity. Gelatin extracts were hydrolyzed by serial protease-treatments in the order of Alcalase, pronase E, and collagenase using a three-step recycling membrane reactor. Fragments arising from the third step were composed of peptides ranging from 0.9 to 1.9 kDa and responsible for ACE inhibitory activity. Catalytically active two peptides were separated by the consecutive chromatographic methods including gel filtration, ion-exchange chromatography, and reverse-phase high performance liquid chromatography. The isolated peptides were composed of Gly-Pro-Leu and Cry-Pro-Met and showed IC50, values of 2.6 and 17.13 muM. respectively. These results suggested that Gly-Pro-Leu would be useful as a new antihypertensive agent.