Sepamin(Chlorpromazine) is today widely used as a tranquilizer in the field of otolaryngology before or after the operation. But the effect on blood coagulation is not known in spite of the importance of blood coagulation and this drug, and the author has studied the effect of Sepamin on blood coagulation with rabbits and human subjects. In the animal experiments, the author used Sepamin (1mg/kg of body weight) intramuscularly, and Lee-White coagulation time was measured before the injection and 30 minutes, 45 min., 60 min., 90 min., and 24 hours after the injection at the room temperature(25℃). The results were noted that only one case out of 10 cases was shortened by the administration of Sepamin, that the rest cases were all prolonged, and that the prolongation of coagulation time was significant. In human subjects, the author used Sepamin (25-50mg) intramuscularly, and Lee-White cogulation time at room temperature before the injection and one hour and 24 hours after the injection was measured, but prothrombin time at 37℃ was measured before the injection and 60 minutes after the injection. The results were noted that only one case out of 9 cases of Lee-White method and also one case out of 8 cases of prothrombin time measurement were shortened, and the other cases were all prolonged. While the sedative action was really excellent. Ne dangerous case was present by the drug during the study. By these studies, Lee-White coagulation time and prothrombin time were prolonged in rabbits and human subjects after the injections of Sepamin although the cases were not many. The reason is not known yet. And the author is still and will try to study this problem deeply with more cases, but the prothrombin level seems to be decreased by the administration of Sepamin. This study is only a part of the author's research works of blood coagulation.