Objective: Canine coronary artery was recently reported to contain a cathepsin like acid optimum enzyme and a kallikrein like alkaline optimum enzyme which cleaved from a crude kininogen preparation a vasodilator uterus contracting substance. The aim of this study was to seek the presence of similar acid optimum enzymes in canine ventricular myocardium and in a large systemic artery, the aorta. Methods: Aqueous canine tissue extracts were tested for the ability at different pHs to release uterus contracting substance (using rat isolated oestrous uterus) from a kininogen preparation. After gel filtration, the extracts were tested for the presence of arginine-amidase activity (substrate: D-Val.Leu.Arg.pNA) and enzymic activity forming bradykinin like immunoreactivity. Tissues were obtained from anaesthetised greyhounds which had been used in control studies and had received no other drug treatment. Results: Ventricular extracts released uterus contracting substance optimally at pH 5.2-5.4, but not at alkaline pH, neither was bradykinin like immunoreactivity formed at alkaline pH. Inhibitor studies and gel filtration showed this activity to be due to a cathepsin-D-like enzyme, molecular weight (MW) 42.6 (SD 0.9) kd, which was an arginine amidase and released bradykinin like immunoreactivity from a plasma kininogen. Aortic extracts showed two pH related peaks of uterus contracting substance formation, at pH 5.2 and (unlike myocardium) at pH 8. Also unlike myocardium, aortic extracts gave two acid optimum kininogenase peaks on gel filtration, with MW 42(4.6) kd and 252(39) kd, respectively. Both peaks released bradykinin like immunoreactivity. Conclusions: Canine aorta contained an alkaline optimum and two acid optimum enzymes, while ventricle contained only a cathepsin-D-like acid optimum enzyme, all of which could form bradykinin like immunoreactivity. The ability of the ventricular enzyme to form a kinin in the slightly acid conditions of myocardial ischaemia may have a protective role.