Placental and myoendometrial pressure-flow relationships were determined in 17 gravid ewes between 29 and 110 gestational days with the use of radioactive-labeled microsphere injections under flow meter guidance. Myoendometrial relationships were consistently curvilinear with convexity toward the flow axis. From 29 to 40 days, caruncular-cotyledonary relationships were similar to myoendometrial ones. After 50 days, the relationship was slightly but significantly curvilinear with convexity toward the pressure axis. From 42 to 50 days, responses were intermediate. The changes in caruncle-cotyledon vascular reactivity correlated with the onset and maturation of interdigitation of fetal and maternal tissues during definitive placentation. A hypothesis for these changes in vascular reactivity is presented.