To address the role of follicular angiogenesis in the determination of ovulatory follicles and the effects of different vascular endothelial growth factor (VEGF) isoforms on follicular angiogenesis and development, mature female rats were treated with an angiogenic inhibitor (TNP-470), and also with VEGF 120 or 164 at different dosages (0.4, 0.8, 4.0 or 8.0 μg/kg body weight) for 3 days during the estrous cycle. Ovarian follicular angiogenesis, the population of large follicles and ovulation were examined. VEGF 120 (0.8 μg/kg) and 164 (8.0 μg/kg) treatments stimulated follicular angiogenesis in the theca interna layer, while TNP-470 treatment showed severe depression of follicular angiogenesis, and completely inhibited ovulation. After administration of VEGF 120 or 164, the number of healthy preovulatory follicles and ovulated oocytes increased significantly, concomitantly with a decrease in the number of atretic preovulatory follicles. The oocytes ovulated had normal fertilizability and developed to term with the same litter size as in the control rats. Our findings suggest that follicular angiogenesis may be a determinant of follicular development during the periovulatory phase, and that VEGF isoforms may play different important roles in regulating follicular angiogenesis.