Abstract: Trichosanthin given im to pregnant rats decreased the plasma progesterone concentration and the level of progesterone and cytosol progesterone receptor in uteri and increased the content of uterine PGF_2α. When the dose response in smooth muscle of pregnant rat uteri was studied after im injection of trichosanthin, the response to 15-Me-PGF_2α and oxytocin was enhanced significantly.Based upon the above experiments it is suggested that: (1) As a result of the deleterious action of trichosanthin on placenta, the PGF_2α content of the uteine tissue was increased. (2) The increase of uterine content of PGF_2α and response to 15-Me-PGF_2α and oxytocin are related to the decrease of plasma progesterone concentration and the level of uterine progesterone and cytosol progesterone receptor.