This study was designed to investigate the effects of inhibition of thecal angiogenesis on follicular development in the marmoset monkey (Callithrix jacchus). To inhibit vascular endothelial growth factor (VEGF), a soluble combined truncated form of the fms-like tyrosine kinase (Flt) and kinase insert domain-containing receptor (KDR) receptor fused to IgG (VEGF Trap R1R2) was administered for 10 d during the follicular phase of the cycle. Changes in angiogenesis and follicular cell proliferation were quantified using immunocytochemistry for bromodeoxyuridine to obtain a proliferation index, CD31 to visualize endothelial cell area, and dual staining to distinguish thecal endothelial cell proliferation. The effects of the treatment on follicular development were assessed by morphometric analyses by measuring follicle diameter, thecal thickness, and a proliferation index for granulosa cells. Follicular atresia was detected and quantified using the terminal deoxynucleotidyltransferase-UTP nick end labeling method. Effects on gene expression of VEGF and its receptors, Flt and KDR, were studied by in situ hybridization. VEGF Trap R1R2 treatment resulted in a significant decrease in thecal proliferation and endothelial cell area, demonstrating the suppression of thecal angiogenesis. The absence of a normal thecal vasculature was associated with a significantly reduced thecal thickness. Antral follicular development was severely compromised, as indicated by decreased granulosa cell proliferation, decreased follicular diameter, and lack of development of ovulatory follicles. Furthermore, the rate of atresia was significantly increased. VEGF expression in granulosa and thecal cells increased after treatment, whereas Flt and KDR expressions in thecal endothelial cells were markedly decreased. These results show that VEGF Trap treatment is associated with the suppression of follicular angiogenesis, which results in the inhibition of antral follicular development and ovulation.