The coronary vasodilator properties of isosorbide dinitrate (ISDN) are well known but the dosage remains empirical. The aim of this study was to construct a dose-response curve to ISDN with respect to vasoconstriction induced by ergometrine. The heart rate, aortic pressure and coronary angiography were analysed before and 3 and 5 minutes after I.V. injection of 0.4 mg of methylergometrine and 3 minutes after intracoronary injection of 5, 15, 60, 240 and 1,000 micrograms of ISDN in 10 patients with an average age of 53.2 +/- 10.8 years (ISDN group). Six other patients with an average age of 56.5 +/- 12.8 years comprised the control group and only received ergometrine. The coronary diameters were measured by quantitative coronary angiography using the CAESAR system of automatic contour detection. Three coronary segments with angiographically normal appearances and a resting diameter greater than or equal to 1.85 mm were analysed in each patient. With respect to the maximal constriction observed 5 minutes after the injection of methylergometrine, the percentage increase in coronary diameter was 9 +/- 7%, 26 +/- 12%, 33 +/- 15%, 38 +/- 14% and 39 +/- 16% after 5, 15, 60, 240 and 1,000 micrograms of ISDN respectively (p less than 0.005 vs control). A plateau effect was observed after a cumulative dose of 80 micrograms and administration of higher doses of 240 and 1,000 micrograms only caused mild nonsignificant additional increase in vessel diameter. In comparison with the control group, the systolic blood pressure only fell significantly with doses greater than 240 micrograms of ISDN (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)