The vasodilatory action of molsidomine was studied by intracoronary injection of its active metabolite, Sin 1. In 10 patients repeat coronary angiography in multiple projections was performed before and 2 minutes after administration of 1 mg of Sin 1, and before and after a second injection 60 minutes later. Contours of obstructed and non-obstructed segments of the left coronary artery were quantitatively analysed with a computer-based angiography analysis system. Immediately after its administration, Sin 1 increased the mean diameters of 44 normal coronary segments by 12% (P less than 0.001). Significant vasodilation (8%) was still observed after 60 minutes. At that time, repeated administration of Sin 1 increased the vasodilation by an additional 14% with respect to the control situation. An increase in obstruction diameter was observed in 6 out of 8 obstructed segments. Mean increase in the minimal obstruction diameter was still 10% after 60 minutes.