The aim of this study was to determine the effectiveness of an intravaginal drug-releasing device commercially produced for use in goats and sheep (CIDR-G device) at inducing estrus in sows. Thirteen Landrace sows were used in the this study, and the sows were randomly divided into three groups: group A (n=5), group B (n=4) and group C (n=4). Two CIDR-G devices were inserted into the vagina of each sow in groups A and B, and one CIDR-G device was inserted into the vagina of each sow in group C. The device was removed from each sow on the seventh day after insertion, and 0.2625 mg of PGF_2α was applied to the vulva of each sow in groups A and C. All five sows in group A and all four sows in group C, the two groups of sows that were administered PGF_2α, showed onset of estrus 3-5 days and 2-4 days after removal of the CIDR-G device, respectively. Three of the four sows in group C showed onset of estrus on the second day after removal of the device. On the other hand, onset of estrus was observed (on day 3 after removal of the device) in only one of the four sows in group B, the group of sows that were not administered PGF_2α. Saliva progesterone concentrations continued to rise in both groups A and C after insertion of the device, and the saliva concentrations of progesterone on day 5 after insertion in these two groups were significantly higher than those just prior to insertion of the device(p<0.05). The number of devices inserted had no effect on the saliva progesterone concentration. The results of this study showed that insertion of a CIDR-G device for a period of seven days followed by application of PGF_2α to the vulva is effective for induction of estrus in the swine.