In the course of the development of the ovarian follicle and differentiation of granulosa cells into corpus luteum (CL), extensive changes in the microvasculature of these structures take place. This suggests the local release of angiogenic factors. In the present work we examined whether a newly described secreted vascular endothelial growth factor (VEGF) is expressed in normal rat ovary by in situ hybridization. Our results demonstrate the expression of VEGF in the CL but not in mural granulosa cells, suggesting a temporal relation between VEGF expression and growth of capillary vessels. The hybridization pattern in the CL was consistent with localization of VEGF message to luteal cells. Expression of VEGF was detected also in cumulus oophorus cells. These findings suggest that VEGF is involved in the process of CL angiogenesis.