This study was conducted to find out the changes of ovarian, placental and fetal weights and periods of pregnancy in rats implanted with progesterone-tube during the reproductive stages. One hundred and thirty-four mature rats, 10~13 weeks old, were offered for this experiment. The animals, which were implanted with silicon tubes filled with progesterone on day 15 of pregnancy, were sacrified at 18, 20, 21 and 22 days of pregnancy. The changes of ovarian, placental and fetal weights and the number of fetuses during late pregnancy were recorded. The results obtained were summarized as follows : 1. After progesterone-tube implantation, ovarian weight reached to a peak value of 92.0$pm$0.9mg at 20 days of pregnancy, there after decreased significantly to 79.5$pm$7.6 and 68.26$pm$4.2mg at 20 and 22 days of pregnancy(P<0.01). 2. The placental weight increased rapidly during 15~18 days of pregnancy in control and progesterone treated rats. A peak value of 447.78$pm$20.9mg was shown at 20 days of pregnancy after progesterone-tube implantation, and in control rats the value decreased significantly to 419.42$pm$11.6 and 404.1$pm$29.3mg at 20 and 21 days of pregnancy(P<0.01). 3. The fetal weights was not shown any significant differences between control and progesterone-tube implanted rats. 4. The number of fetuses in control rats were 14.75$pm$0.4 at 8~10 days of pregnancy and 13.5$pm$0.3 and 13.25$pm$0.4 at 12 and 20 days of pregnancy. 5. The significant difference in periods of pregnancy was appeared between progesterone-tube implanted(27.3$pm$0.3 days) and control(22.1$pm$0.3 days)rats(P<0.01).