The presence of an acid optimum (pH 6) enzyme capable of generating a spasmogenic, vasodilator polypeptide from human plasma kininogen has been demonstrated in dog coronary arteries, veins and in the wall of the left ventricle. This enzyme also cleaved the tripeptide kallikrein substrate Val-Leu-Arg-pNA. Highest amounts were present in the coronary arteries. Gel filtration (Sephacryl S-300) of coronary artery extracts gave a peak for this acid optimum enzyme of 38, 300 +/- 800 Daltons. Its activity was inhibited by pepstatin but not by aprotinin or soya bean trypsin inhibitor. This enzyme, which is similar to a cathepsin, may play a role in the processing of peptide hormones in the heart and coronary vessels.