1. The role of nitric oxide in the coronary circulation under basal conditions and when exposed to various vasodilator stimuli was studied in instrumented, anaesthetized goats, by examining the action of inhibiting endogenous nitric oxide production with NG-nitro-L-arginine methyl ester (L-NAME). 2. In 12 goats, left circumflex coronary blood flow (electromagnetically measured), systemic arterial blood pressure and heart rate were continuously recorded. L-NAME (3-4, or 8-10 mg kg-1 injected i.v.) decreased resting coronary blood flow by 20 and 28%, increased mean arterial pressure by 23 and 30% and increased coronary vascular resistance by 47 and 65%, respectively, without affecting heart rate, or blood gases or pH. These haemodynamic effects were reversed by L-arginine (200-300 mg kg-1 by i.v. injection, 5 goats). 3. Acetylcholine (0.001-0.1 micrograms), sodium nitroprusside (0.01-0.3 mg), and diazoxide (0.1-3 mg), injected intracoronarily in 6 goats, produced dose-dependent increases in coronary blood flow; sodium nitroprusside (0.1-0.3 mg) also caused hypotension and tachycardia. 4. During the effects of L-NAME, the coronary vasodilatation to acetylcholine was attenuated, to sodium nitroprusside was increased, and to diazoxide was unaffected, in comparison with control conditions. The hypotensive effects of sodium nitroprusside were also increased during treatment with L-NAME. 5. Graded coronary hyperaemic responses occurred after 5, 10 or 20 s of coronary occlusion. The magnitude of hyerpaemia for each occlusion duration was increased during treatment with L-NAME, in comparison to control.6. The results suggest: (a) endogenous nitric oxide is involved in regulation of coronary circulation by producing a basal vasodilator tone, (b) acetylcholine-induced coronary vasodilatation is mediated, in part, by nitric oxide, and (c) inhibition of basal endogenous nitric oxide production induces supersensitivity of coronary vessels to nitrovasodilators and enhances hyperaemic responses after short periods of ischaemia of the myocardium.