Indirect evidence in the nonhuman primate and human suggests that angiogenesis and regulators of angiogenesis such as vascular endothelial growth factor (VEGF) may play an active role in cyclic folliculogenesis. Indeed, the follicle selected for maturation and ovulation possesses a denser microvascular network, and VEGF messenger ribonucleic acid and its protein have been identified in granulosa cells of the developing follicle during the mid- and late follicular phases, with a more intense signal in the mature follicle. The objective of this study was to obtain direct evidence in the nonhuman primate for an active role of VEGF in follicular growth and maturation by studying the effect of VEGF-blocking antibodies in this process. After documenting two normal ovulatory cycles, female rhesus monkeys (n = 7) received iv injections of anti-VEGF antibodies (0.5 mg) twice on successive days in the late follicular phase. Three monkeys also received nonspecific goat IgG (0.5 mg) twice on successive days in the late follicular phase. Daily measurements of estradiol, progesterone, LH, and FSH were obtained during the two control cycles, the anti-VEGF treatment and posttreatment cycles, and the IgG treatment cycle. Anti-VEGF antibody administration significantly lengthened the follicular phase in six of seven monkeys to 17.8 ± 1.7 vs. 10.0 ± 0.7 and 9.8 ± 0.6 in control cycles and 10.7 ± 0.3 days (mean ± se) in IgG-treated cycles. The expected late follicular phase rise in estradiol, as documented in the control cycles (day 0, 96.1 ± 6.0; day 1, 125.5 ± 20.0; day 2, 165.5 ± 24.9; day 3, 183.8 ± 11.0 pg/mL), was interrupted by anti-VEGF antibody treatment (99.3 ± 5.0, day 0, preinjection control) to 63.3± 12.2 (day 1), 48.5 ± 8.7 (day 2), and 57.6 ± 9.0 (day 3). Mean FSH levels were significantly increased by day 2 of anti-VEGF antibody treatment. After a variable delay, estradiol concentrations increased to reach a preovulatory peak in all anti-VEGF-treated animals, followed by ovulation, normal luteal function, and a normal posttreatment cycle. The data clearly demonstrate that short-term inhibition of angiogenesis with an anti-VEGF-blocking antibody during the later growth phase of the dominant follicle interferes with normal follicular development. Persistence of estradiol secretion and delayed resumption of its rise also suggest recovery of the follicle. We conclude that the angiogenic regulator VEGF is a crucial component in the process of follicular growth in the primate.