Homotaurine (HT) elicited a cardio-stimulant action at high concentration on isolated frog hearts, guinea-pig atria and guinea-pig hearts, though it brought about no actions in the heart lung preparation of dogs. In rabbits, HT showed a mild cardiac-stimulating as well as a hypotensive effect. The hypotensive effect was likewise observed in rats and was shown to be more potent in experimental renal hypertensive rats than that in normal rats. These cardiovascular effects were decreased when it was administered repeatedly. The cardio-stimulant action of HT was retained after administration of adrenergic α and β-blocking agents, and, although HT showed a mild protective effect against the cardio-depressant action of dinitrophenol and large doses of CaCl2, cardiac action of autonomic drugs, CaCl2 and KCl were not affected by HT. From these results, it has been suggested that HT has a hypotensive and a cardiotonic effect, its cardiac effect being due to its direct action.