To investigate the influence of ovariectomy on the physiological process of neovascularization of ovarian follicles, we harvested follicles with a theca interna at 48 h after pregnant mare's serum gonadotropin (PMSG) treatment of Syrian golden hamsters, for subsequent transplantation onto striated muscle tissue of chronically implanted skinfold chambers of non-ovariectomized PMSG-synchronized hamsters and bilaterally ovariectomized hamsters. Non-ovariectomized non-PMSG-treated animals served as controls. During a 2 week period after transplantation, neovascularization of these freely transplanted grafts was quantified in vivo by assessment of the newly developed microvascular follicular network, its microvessel density, the diameter of microvessels and their volumetric blood flow using fluorescence microscopic techniques. At day 3 after transplantation, capillary sprouts could be observed in all groups studied, finally developing a complete glomerulum-like microvascular network within 5-10 days. In ovariectomized animals, however, vascularization of follicular tissue was found to be accelerated and enhanced when compared with the follicles in both groups of non-ovariectomized animals. This was associated with significantly higher capillary blood perfusion, which may in part represent the graft's adaptive response to pro-angiogenic stimuli due to elevated gonadotropin levels, but might in particular be mediated by gonadotropin-induced release of vasoactive substances. Interestingly, small preantral follicles lacking a theca interna failed to vascularize in either of the groups. In conclusion, the study demonstrates that ovariectomy improves neovascularization and microcirculation of freely transplanted ovarian follicles.