This stduy was conducted to find out the changes of estradiol-17$eta$ and progesterone levels in the plasma of Cheju mares during gestatation period and before and after parturition with 12 heads of Cheju ponies. The results are as follows : 1. The estradiol-17$eta$ and progesterone levels on the 1~4 months showed 61.6~134.2pg/ml and 7.74~9.20ng/ml respectively, but the estradiol-17$eta$ levels rapidly increased to 426.4~1772.9pg/ml, and the progesterone levels decreased to 1.42~5.43ng/ml on the 5~10 months of gestation period. 2. The progesterone levels of the pregnant and the non-pregnant mares showed 4.80ng/ml and 0.04ng/ml in winter, respectively, but both of the two appeared 1,211.5pg/ml and 99.4pg/ml of estradiol-17$eta$, respectively. Therefore the diagnosis of pregnancy might be more accurate with the levels of estrdiol-17$eta$ in autumn. 3. The progesterone levels showed 3.20ng/ml the day before parturition and less than 1ng/ml on 2 days after parturition. The estradiol-17$eta$ levels, showing the rising trend, and 90.1~162.4pg/ml up to 10 days.