In order to examine the effects of PGF2$alpha$ on intervals from weaning to estrus and serum progesterone levels seventeen crossbred primiparious sows were randomly alloted to two groups. One group was injected intramuscularly on the day of weaning with 10 mg PGF2$alpha$ (10mg/2ml, Lutylase). The other group was treated with saline as a control. Serum progesterone concentrations were determined at 24 hour intervals for 12 days after weaning. A serum progesterone level in PGF2$alpha$-treated group was reached to the lowest level(1.19$pm$0.38 ng/ml) on day 3 after weaning and remained low(1.26~1.43ng/ml) thereafter. Whereas, the control group showed the lowest level of progesterone on day 4 after wearing, then showed a rapid increase up to 5.02$pm$0.38ng/ml on day 8 and a rapid decrease was followed. The PGF2$alpha$ treated group showed an interval from weaning to estrus(5.2$pm$0.8 days) approximately 2 days shorter than the control(7.4$pm$3.0 days)(p<0.05).