Abstract: Isolated uterine preparations obtained from virgin and pregnant guinea-pigs were used. Experiments were carried out according to Magnus method with low-calcium Locke's solution at a temperature of 32℃. 10μg/ml of rivanol usually caused a conspicuously excitant action in both virgin and pregnant uterine preparations. The uterine excitation was blocked effectively by prior treatment of the preparations with 40μg/ml of indomethacin for 3 hours. Treatment of the uterine segments with 2μg/ml of chlorpromazine for half an hour could also block the excitant response. The probable mechanism of action of rivanol was discussed.