Abstract
The study was carried out to find out the changes of hormone levels in blood serum and milk of Holstein cows during the estrous cycle. The progesterone, estradiol-$17{\beta}$ from the blood serum and milk samples were assayed by radioimmunoassay methods. The results of this study were summarized as follows; 1. The progesterone levels in blood serum during the estrous cycles began to decline rapidly at 2 days before estrus, decreased to $0.27{\pm}0.18ng/ml$ at on the day of estrus, and reached a peak mean level of $3.33{\pm}0.47ng/ml$ at 15 days after estrus. 2. The progesterone levels in milk during the estrous cycles began to decline rapidly at 2 days before estrus, decreased to $0.80{\pm}0.18ng/ml$ on the day of estrus, and increased a peak mean level of $3.80{\pm}0.36ng/ml$ at 15 days after estrus. 3. The estradiol-$17{\beta}$ levels in blood serum during the estrous cycles showed a peak mean level of $9.79{\pm}1.72pg/ml$ on the day of estrus, and decreased from $4.79{\pm}1.82pg/ml$ to $5.73{\pm}0.96pg/ml$ at luteal phase. 4. The estradiol-$17{\beta}$ levels in milk during the estrous cycles showed a peak mean level of $36.80{\pm}2.04pg/ml$ on the day of estrus, and decreased from $18.93{\pm}0.84pg/ml$ to $19.50{\pm}1.12pg/ml$ at luteal phase. 5. During 20 to 25 days after artificial insemination, the accuracy of pregnancy diagnosis from the blood serum progesterone levels were 87.5% for non pregnant cows (<2.0ng/ml), and 83.3% for pregnant cows ($${\geq_-}$$3.0 ng/ml). The accuracy of pregnancy diagnosis from the milk progesterone levels were 75.0% for non-pregnant cows (<2.4 ng/ml), and 94.4% for pregnant cows ($${\geq_-}$$3.2 ng/ml).