Abstract—Two enzymes that selectively hydrolyse kinins at pH 7.5 were obtained in partially purified form from the supernatant fraction of homogenates of previously frozen rabbit brain by gel filtration on Sephadex G‐100. The enzymes were detected and their activity estimated by bioassay with the isolated guinea pig ileum The products of the enzymic reactions were identified by high voltage electrophoresis at pH 3.5 and by the determination with the amino acid analyser of the amino acids released from the kinins.One enzyme, kinin‐converting enzyme, catalyses the hydrolysis of kinin‐10 (Lysbradykinin) and kinin‐11 (Met‐Lys‐bradykinin) into kinin‐9 (bradykinin). It also hydrolyses the aminoacyl‐8‐naphthylamides of methionine, lysine, arginine and leucine. The conversion of kinin‐10 to kinin‐9 was inhibited by puromycin (Ki3.5 × 10−5M) These properties are similar to those of brain arylamidases described in the literature.Kininase, the second enzyme, inactives kinins 9, 10 and 11 by peptide‐bond hydrolysis. Similar rates of release of arginine and phenylalanine were observed for the three kinins, suggesting that kininase acts at the carboxy‐terminus of these peptides.Our results suggest that brain contains proteases which apparently selectively metabolize polypeptide hormones that exert definite pharmacological effects on the central