To investigate the possible role of the vasculature in the local regulation of corpus luteum (CL) function, we determined the densities of capillaries and large blood vessels in the center of the bovine CL during the estrous cycle and following prostaglandin (PG) F2alpha-induced luteolysis. The CLs at the early (Days 2-3 post-ovulation), developing (Days 5-7), mid (Days 8-12), late (Days 15-17) and regressed (Days 19-21) stages were collected. In addition, the CLs were collected by transvaginal ovariectomy from 12 cows (Day 10 after ovulation), i.e., non-treated (n=3, 0 h, control), at 0.5 (n=3), 2 (n=3) and 12 h (n=3) after injection of a luteolytic dose of PGF2alpha. Immunohistochemical staining with von Willebrand Factor (specific for endothelial cells that are found in both types of blood vessels) revealed that the density of the luteal blood vessels was significantly higher at the developing and late luteal stages (P<0.05) than at the other stages, whereas the number of larger blood vessels (those stained with alpha-smooth muscle actin) was higher at the late and regressed luteal stages (P<0.05) than at the other stages. Furthermore, both the density of blood vessels and the number of blood vessels with smooth muscle were significantly higher in the CLs obtained at 2 h and 12 h after PGF2alpha administration (P<0.05) than in those without PGF2alpha treatment. These results suggest that the number of blood vessels with smooth muscle per unit area in the regressing CL increased as a result of losing steroidogenic cells and capillaries. The overall results demonstrate that the capillaries disappeared earlier than the large blood vessels during structural luteolysis and suggest that the loss of capillaries in the CL results in a reduced supply of nutrients and oxygen to luteal cells followed by cell death.