The lining of blood vessels (endothelium) releases a variety of substances that are capable of relaxing smooth muscles of blood vessel wall. Certain conditions might lead to an abnormality in production of these relaxing substances, an important member being nitric oxide. In this study we investigated whether nitric oxide released from the heart arteries (coronary circulation) was affected by development of atherosclerosis or hardening of the blood vessels. Patients with normal coronary arteries without risk factors for atherosclerosis were compared with those with established atherosclerosis. By using L-NMMA, an agent that blocks production of nitric oxide, we were able to demonstrate that the availability of nitric oxide from the coronary circulation is reduced in patients with atherosclerosis affecting the heart arteries.