These studies were carried out to establish an effective and practical system for commercialization of embryo production techniques by analyzing several factors influencing in vivo embryo production in Korean native cattle. In vivo embryos were flushed 226 times from 128 donors. The results obtained in studies on the factors influencing in vivo embryo production were as follows : 1. There were no significant differences in the number of total recovered, fertilized, transferable and freezable embryos among the hormone doses(FSH-P, 28∼34mg; SUPER-OV, 75IU) used for superovulation. However, over 30mg doses of FSH-P showed a slightly higher effect than others. 2. There were slight decrease in the number of fertilized, transferable and freezable embryos in 3 times repeated superovulation. But there were no significant differences among 1, 2 and 3 times repeated superovulations. 3. Age of donors did not affect the number of transferable and freezable embryos, but the number of fertilzed embryos were highest in 2∼3 years old donors and were lowest in 8∼9 years old donors(P<0.05). 4. Season had a significant effect on the production of embryos(P<0.05). the embryo production, and the number of fertilized, transferable and freezable embryos were most effective in summer and follow by spring, autumn and winter. 5. The number of transferable and freezable embryos was highest in donors flushed at 7∼8 days after estrus(P<0.05).