Effects of new inhibitors of thromboxane synthetase, (E)-3-[(1-imidazolmethyl)phenyl]-2-propenoic acid and (E)-3-[4-(pyridylmethyl) phenyl]-2-methyl-2-propenoic acid on cyclical reduction of flow in the partially constricted coronary artery were examined in anesthetized beagle dogs. Intravenous injections of both agents with a dose of 20mg/Kg eliminated the cyclical reduction induced by constriction in the majority of experiments. However, they failed to eliminate the cyclical reduction induced by indomethacin. Indomethacin-induced reduction was eliminated by prostaglandin I2 in all experiments. It is suggested that thromboxane A2 acted as an accelerator in the cyclical reduction of coronary flow induced by coronary constriction, but did not in the reduction induced by indomethacin