The study was carried out to find out the changes of the sex hormone levels in the milk of Holstein cows during the reproductive stages such as the estrous cycle, pregnancy and periparturient period. The FSH, LH, estradiol-17$eta$ and progesterone from the milk samples were assayed by radioimmunoassay methods. The results of this study were summarized as follows: 1. The levels of progesterone and estradiol-17$eta$ were similar among inter-quarters, but they were higher in after milking than before milking times, with no statistical significance. 2. The milk progesterone levels during the estrous cycles reached a peak mean level of 3.55$pm$0.26ng/$mell$ at 15 days after estrus and they did not show any differences among the length of estrous cycles. The estradiol-17$eta$ levels during the estrous cycles showed a peak level of 36.40$pm$2.38pg/$mell$ at estrus, and decreased(17.20$pm$0.46 pg/$mell$ to 18.65$pm$1.26pg/$mell$) at luteal phase. 3. The FSH levels during the estrous cycles ranged from 2.25$pm$0.23mIU/$mell$ to 4.35$pm$0.24mIU/$mell$ showing significant changes. The LH levels during the estrous cycles gradually increased and remained a peak level of 10.90$pm$0.36mIU/$mell$ from 20 to 25 days after estrus. 4. The progesterone levels during the pregnancy were decreased from 30 to 60 days after artificial insemination, and therafter continuously increased until 240 days. The estradiol-17$eta$ levels during the pregnancy were 24.56$pm$1.19pg/$mell$ at day 30 after artificial inseminaton, and increased rapidly until 180 days. The levles were agagin decreased by 26.17$pm$3.03pg/$mell$ until 210 days and markedly increased by 68.00$pm$8.70pg/$mell$ until 240 days. 5. The prolactin levels during the pregnancy were 31.27$pm$2.31ng/$mell$ and 42.60$pm$2.37ng/$mell$ at day 150 and 240 after artificial insemination respectively. The LH levels during the pregnancy reached a peak of 27.47$pm$7.90mIU/$mell$ at day 30 after artificial insemination, and thereafter gradually decreased. 6. The progesterone levels during the periparturient period reached a peak of 4.61$pm$0.34ng/$mell$ at day 3 prepartum, and thereafter gradually decreased, and showed 2.05$pm$0.60ng/$mell$ at day 7 postpartum. The estradiol-17$eta$ levels during the periparturient period showed high level from 207.23$pm$6.04pg/$mell$ at day 1 prepartum to 239.90$pm$13.90pg/$mell$ at day 2 prepartum, and thereafter began to decline and reached 51.87$pm$1.72pg/$mell$ at by 7 postpartum. 7. The prolactin levels during the periparturient period showed relatively higher level at the time of parturition. The LH levels during the periparturient period rnage from 6.32$pm$0.32mIU/$mell$ to 13.90$pm$1.37mIU/$mell$ showing significant changes. 8. The progesterone levels(4.6$pm$0.8ng/$mell$) of the pregnant cows were significantly higher than those (1.84$pm$1.4ng/$mell$) of nonpregnant cows. The cows of artificial insemination from 61 to 90 days after parturition showed higher progesterone levels. 9. During 20 to 25 days after artificial insemination, the accuracy of pregnancy diagnosis from milk progesterone levels were 94.4% for nonpregnant cows(<2.3ng/$mell$), and 75.0% for pregnant cows( 3.2ng/$mell$). The average overall accuracy of pregnancy prediction for nonpregnant and pregnant cows 83.3% 10. The results obtained this study suggest that the understan..