Abstract: Water extract of Cortex toosendan (1∶1) was given by oral admistration to the pregnant mice on day 7 in order to study its toxicity. Embryo livability was examined. The natural killer cells in the uterine decidua were detected by immunohistochemistry. The Th1 cytokines IFN-γ and TNF-α in the uterine lysates were analyzed by ELISA. The results showed that Cortex toosendan is toxic to embryo, and that the toxicity is related to the enhancement of Th1 responses at the maternal-fetal interface, with the increase of NK cell counts, IFN-γ and TNF-α concentrations in pregnant uterus.