This study was undertaken to determine the effects of oral use of MGA on estrus synchronization and fertility at synchronized estrus in Korean Native cattle. One ㎎ of MGA per head per day was fed for 10 days to 10 normal and 10 repeated cows begining at day 4,5,6,7,8,10,12,14,15 and 16 post-estrus. Development of estrus, follicle formation in the ovary and conception rate at first synchronized estrus were checked by palpation. In any estrus before and after MGA treatment the right ovary showed higher follicle formation frequency than the left ovary. The cow developed estrus at 1 to 5 days, 6 to 10 days and more than 10 days after the last feeding of MGA was 50%, 25% and 25% respectively. Effective induction of synchronized estrus was made in the cow begining MGA at late luteal stage rather than at early and functional stages. The cow treated MGA developed estrus at average 7.6 days after the last feeding. Four in 10 normal cows and 2 in 10 repeated cows were respectively conceived :at first synchronized estrus following treatment. There was some possibilities to be fertile in the repeated cow by treatment of MGA. This investigation was supported by grant of MGA from the Upjohn Company, Kalamazoo, Michican, U.S.A.. I sincerely thank Mr. Ewald Manager of Upjohn Company in Korea, for his efforts of supplying MGA.