This study was conducted to find out the changes of progesterone and 20$alpha$-dihydroprogesterone(OHP) levels in the serum of female rats by progesterone-tube implantation during late pregnancy. One hundred and twenty rats, 10-13 weeks old, were offered for this experiment. Blood samples were taken from the rats on 18, 20, 21 and 22days of pregnancy. The rats were implanted with silicon tubes filled with pregesterone on day 15 of pregnancy. The progesterone and 20$alpha$-OHP in serum were assayed by radioimmunoassay. The results obtained are summarized as follows : 1. The progesterone levels in rats of control group showed 126.36$pm$20.19ng/ml on 18days and gradually decreased to 69.3$pm$11.9, 29.2$pm$1.8ng/ml 20 and 21 days. 2. The progesterone levels after progesterone-tube implantation showed 120.1$pm$8.5, 59.01$pm$3.1, 85.33$pm$17.9 and 62.9$pm$6.1ng/ml on 18, 20, 21 and 22days of pregnancy, respectively. 3. The 20$alpha$-OHP levels in rats of control group showed 68.5$pm$8.0ng/ml on 18 days and gradually increased to 139.9$pm$3.7 and 141.4$pm$6.7ng/ml on 21 and 22days. 4. The 20$alpha$-OHP levles after progesterone-tube implantation showed 108.5$pm$32.7 and 106.4$pm$10.6ng/ml on 18 and 20days and increased to reach the peak level at 21days(225.01$pm$9.9ng/ml) and rapidly decreased on 22days(85.9$pm$10.7ng/ml).