The bioavailability of nitric oxide (NO) in the human coronary circulation at rest and after acetylcholine (ACH)-induced vasodilation was investigated in 32 patients with angiographically normal coronary arteries. The effects of intracoronary L-NG monomethyl arginine (L-NMMA) were investigated at rest and after ACH, sodium nitroprusside, and adenosine. L-NMMA (64 mumol/min) increased resting coronary vascular resistance by 22% (P < 0.001), reduced distal epicardial coronary artery diameter by 12.6% (P < 0.001), and inhibited ACH-induced coronary epicardial and microvascular vasodilation. These effects were reversed with intracoronary L-arginine. L-NMMA did not inhibit dilation in response to sodium nitroprusside and adenosine. 23 patients were exposed to one or more coronary risk factors. The vasoconstrictor effect of L-NMMA on the epicardial and microvessels was greater in patients free of risk factors: Coronary vascular resistance was 36% higher in patients without risks, compared to 17% higher in patients with risks (P < 0.05). Both epicardial and microvascular dilator effects of ACH were greater in patients without risk factors, and the inhibition of these effects by L-NMMA was also greater in patients without risk factors. Thus: (a) NO contributes importantly to resting epicardial and coronary microvascular tone, (b) coronary vascular dilation in response to ACH is predominantly due to increased production of NO, and (c) despite the absence of angiographic evidence of atherosclerosis, exposure to coronary risk factors is associated with reduced resting and stimulated bioavailability of NO from the human coronary circulation.