Ditazole (4,5-diphenyl-2-bis-(2-hydroxyethyl)-aminoxazol) is a new drug shown to inhibit prostaglandin release from rat platelets. The present study indicates that ditazole, at doses similar to those inhibiting prostaglandin formation in rat, inhibits in vivo collagen--but not ADP-induced rat platelet aggregation. In addition, ditazole does not prolong the bleeding time in rats, but even tends to shorten it. This effect could be due to the inhibition of prostaglandin formation at the side of the vascular injury produced to induce bleeding.