We reviewed recent in vivo studies of the real-time changes in the vasculature of the follicle wall during selection of the dominant follicle as well as during ovulation in cows. Changes in follicle diameter and vascularity were determined by transrectal ultrasonography. Blood flow within the walls of the two largest follicles was detected at the time of wave emergence (largest follicle=5 mm in diameter). Before selection of a follicle (largest follicle <8.5 mm in diameter), the degrees of vascularity of the two largest follicles were not significantly different. After the largest follicle reached a diameter of 10 mm, the vascularity of the largest (dominant) follicle was higher than that of the second largest (subordinate) follicle. In the preovulatory follicle, follicular vascularity gradually increased, and as ovulation approached, the LH-surge induced an increase in blood flow within the follicle wall. The above results suggest that maintenance of follicular vasculature and appropriate blood supplies to follicles are essential for establishment of follicular dominance. Consequently, only a dominant follicle with high vascularity may have a chance to reach final maturation and acquire ovulatory capacity.