This paper was aimed to explore the inhibition effect of cardiotonic pills (CP) on venous thrombosis induced in rat mesentery by photochemical stimulation (PS). Male Sprague-Dawley rats were anesthetized with urethane. Thrombosis was induced in the mesenteric venule by PS with photosensitizer hematoporphyrin and an ultraviolet beam. The rats were divided into three groups: control (n = 6), PS (n = 6) and PS + CP group (n = 6) where CP solution (0.4 g/kg.b) was administrated orally 60 min before PS. Microcirculatory disturbances in the mesentery were observed under an inverted microscope with a color video-camera. Based on the recorded images, the development of thrombosis was evaluated in term of time of thrombosis appearance (T(a)) and area ratio of thrombus/vessel (AR). The expression of adhesion molecule (CD31) of platelet was examined in blood taken from the abdominal aorta, using flow cytometry. The T(a) was approximately 10 seconds after PS in PS group, but it was approximately 20 seconds in PS + CP group. The AR in PS + CP group was significantly reduced, compared to that in PS group, during the period of the observation. The CD31 expression was not changed in both groups, while positive cells were significantly increased in the number. It is suggested that CP might suppress thrombus development under the interaction of platelet with endothelium.