To elucidate the mechanism of anti-ischaemic and anti-anginal action of angiotensin-converting-enzyme inhibitors, a randomized double-blind study was undertaken in 30 consecutive patients (27 men, 3 women; mean age 58 [28-70] years) with stable angina and at least 50%, angiographically well demonstrated, stenosis of one of the main coronary artery branches. They received an intracoronary infusion of either 0.5 mg captopril (n = 16) or of a placebo (n = 14) to see whether in this form of application the drug could cause an acute dilatation of a coronary stenosis. The diameter before captopril administration was 1.40 +/- 0.63 mm, while 1, 5 and 10 min after infusion it was 1.49 +/- 0.58 mm, 1.30 +/- 0.54 mm and 1.41 +/- 0.59 mm (not significant). There was also no significant difference between captopril and the placebo. The absence of effect with captopril may be due to insufficient liberation of endothelium-derived relaxing factor in an arteriosclerotic coronary segment.