We investigated the antithrombotic activity of Z-335, a new thromboxane A2 receptor antagonist, using experimental thrombosis models, and also tested its effect on the rat tail bleeding time. Z-335 (0.1, 0.3, and 1 mg/kg, p.o.) dose-dependently prevented the occurrence of arachidonic acid-induced pulmonary thromboembolism in mice. During photochemically induced thrombosis in the femoral artery of guinea pigs, Z-335 (0.3, 1, and 3 mg/kg, i.v.) dose-dependently prolonged the time required to form thrombi. Moreover, Z-335 (10 mg/kg/day, p.o.) strongly suppressed lauric acid-induced hind limb injury in rats. Z-335 (0.3, 3, 30, and 300 mg/kg, p.o.) did not prolong the tail bleeding time in rats. These results strongly suggest that Z-335 ameliorates experimental thrombosis without prolonging the rat tail bleeding time, and may therefore be a useful antithrombotic drug.