The objective of this study was to determine changes in circulating progesterone and insulinlike growth factor-I(IGF-I) concentrations during estrous cycles in Hanwoo. The serum progesterone and IGF-I concentrations during the estrous cycles in the apparently healthy normal cycles and freemartins of 670 days old were measured with a commercial Radio Immuno Assay(RIA) kit. Serum progesterone concentration was low from Day 1 to 3(about 0.25ng/㎖) and then increased gradually to 9.3ng/㎖ by Day 17, and decreased dramatically by Day 21 of the estrous cycle. However, serum progesterone concentration of freemartins was almost undetectable during the same periods of normal Hanwoo. Serum IGF-I concentrations from Day 3 to 16 of normal estrous cycle were higher than those of early luteal stage( Day 1∼2) and luteal regression stage (Day 19∼20). The lowest serum IGF-I concentration was observed on Day 21. In the freemartin of Hanwoo, serum IGF-I concentrations were 1/4 of those in the normal cycles. It is suggested that the serum progesterone concentration is closely related to the serum IGF-I concentration in Hanwoo.