Nitroglycerin potently dilates large coronary arteries but has minimal effect on coronary resistance vessels. Thus, nitroglycerin is not likely to be converted to vasoactive intermediates in small coronary microvessels (less than 100 microns diameter). Because nitroglycerin biotransformation may involve sulfhydryl groups, the effect of L-cysteine on the dilation caused by nitroglycerin (1 nM to 10 microM) was examined in small (80-100 microns in diameter) and large (190-300 microns in diameter) porcine coronary microvessels. Vessels were pressurized in a no-flow state and preconstricted with acetylcholine, then visualized by means of an in vitro microvessel imaging apparatus. Under control conditions, nitroglycerin caused potent dilations of large coronary microvessels, while having minimal effects on small coronary microvessels [peak relaxations 90 +/- 3 vs. 20 +/- 7% (mean +/- S.E.M.) of preconstricted diameter, respectively]. L-Cysteine (100 microM) markedly enhanced relaxations of small coronary microvessels (peak relaxation 96 +/- 2%), while having no effect on relaxations of large coronary microvessels. Ethacrynic acid, which alkylates sulfhydryl groups, markedly inhibited relaxations of large coronary microvessels (peak relaxation = 31 +/- 9%) yet had a minimal effect on the small relaxations of small coronary microvessels to nitroglycerin. Thus, when sulfhydryl groups are made available, small coronary microvessels are markedly responsive to nitroglycerin. This suggests that small coronary microvessels normally lack the reductive capacity to convert nitroglycerin to its active metabolites. Prussian blue stains of sulfhydryl groups in large and small coronary microvessels were qualitatively similar. Thus, small coronary microvessels do not lack sulfhydryl groups but may be deficient in a critical pool necessary for nitroglycerin biotransformation.