The intraovarian axis between granulosa cells and thecal cells is regulated by locally produced autocrine and paracrine factors. Until now, microvascular endothelial cells (MVEC) have not been included in such studies. Bovine granulosa cells from medium-sized antral follicles were plated at low density into the lower compartment of 24-well-culture plates on day 0. MVEC derived from bovine corpus luteum were seeded on appropriate inserts and placed as the upper compartment on day 1. Control granulosa cell cultures and MVEC co-cultures were maintained in serum-containing medium. On day 21, control cultures displayed an epithelioid monolayer and the coculture displayed a multilayer. Histochemical staining for 3 beta-HSD activity and for the lipid droplet stain with the fluorescent dye Nile Red were strong, suggesting augmented steroidogenesis in the multilayer. Yet the progesterone levels of supernants corrected for 10,000 cells were similar in monolayers and in multilayers. Co-cultures contained approximately three times more granulosa cells than control cultures as evaluated with a Coulter counter. Additionally, the occurrence of dead cells was quantified with the fluorescent DNA stain, ethidium homodimer, in 11-day-old control cultures and MVEC co-cultures which were deprived of serum, MVEC, or both for an additional 40 h. Serum and MVEC suppressed the occurrence of granulosa cell death. It is concluded that MVEC produce survival factors for the growth and maintenance of granulosa cells.