1. BM 12.434 was compared with a known β-adrenoceptor-blocking agent, metipranolol, and a combination of metipranolol and isosorbide-5-mononitrate in healthy volunteers. BM 12.434 and metipranolol were given in equi-effective β-adrenoceptor-blocking doses (reduction of exercise-increased pulse-pressure product). 2. Responses of the cardiovascular system were determined by non-invasive methods. 3. BM 12.434 increased the peripheral resistance less than metipranolol. 4. After BM 12.434, in contrast to metipranolol, both arterial flow and the venous capacity showed significant increase. 5. The combination of isosorbide-5-mononitrate and metipranolol differed from metipranolol alone mainly by the effect on venous capacity, which showed a slight increase. 6. BM 12.434 is a β-adrenoceptor-blocking agent whose additional actions on the veins and arteries are potentially useful for the treatment of both coronary heart disease and of arterial hypertension.