The development of the corpus luteum (CL), which involves angiogenesis, is essential for the establishment of early pregnancy. We investigated the roles of the prostaglandin synthases cyclooxygenase (COX) I and COX-II in angiogenesis and progesterone production in the newly formed CL, using inhibitors of the COX enzymes and the gonadotropin-induced pseudopregnant rat as a model. Injection of indomethacin, a nonselective COX inhibitor, on the day of ovulation and the following day decreased serum levels of progesterone, as did injection of the selective COX-II inhibitor NS-398. In contrast, a selective COX-I inhibitor, SC-560, had no effect on serum progesterone concentrations. None of the inhibitors had any effect on the weight of the superovulated ovaries or on the synthesis of progesterone by cultured luteal cells. To determine whether changes in angiogenesis are responsible for the decrease in progesterone synthesis, we measured hemoglobin and CD34 levels in luteinized ovaries following injection of COX inhibitors and measured the relative frequency of cells positive for platelet-endothelial cell adhesion molecule as a specific marker for endothelial cells. All of these parameters were reduced by the COX-II inhibitors, suggesting that changes in the vasculature are responsible for the decrease in serum progesterone. Histological examination of ovarian corrosion casts indicated that NS-398 inhibited the establishment of luteal capillary vessels following the injection of hCG. The results are consistent with the hypothesis that the activity of COX-II is associated with the formation of functional CL via its stimulation of angiogenesis.