OBJECTIVE: To observe preventive and therapeutic effects and the mechanism of actions on Zishen Yutai pill combined with progesterone on threatened abortion in rats. METHODS: After pregnancy, 50 SPF female SD rats were selected and divided into control group, model group, progesterone group, Zishen Yutai pill group, and progesterone plus Zishen Yutai pill group (combination group), with 10 rats in each group. The rats of control group and abortion model group were lavaged with 2?mL/kg normal saline on pregnancy day 1 for continuous 10 days. Rats in Zishen Yutai pill group were given 1.575?g/kg/d of Zishen Yutai pill for intragastric administration for continuous 10 days. Rats in progesterone group were given intramuscular injection treatment of 0.1?mL/d progestin, continuous for 10 days. Rats in combination group were given injection therapy of aqueous solution of Zishen Yutai pill for continuous 10 days, and other treatments were the same as previous two groups. Abortion model were established then and live births, numbers of abortion and average rate of abortion were compared between the five groups. Peripheral blood was collected to detect the estradiol (E2) and progestational hormone (P), and obtain ratio of Th1/Th2 cytokines (IL-2, INF-γ, IL-4, IL-10). RESULTS: Significant more live births of rats were found in the control group compared with other four groups (P??0.05) but these levels in the control group and Zishen Yutai pill group were significant lower than that of progesterone group and joint group (P?