Accurately timed gestations in Rhesus monkeys are used during the second and third trimester. Acute experiments are performed under anesthesia and aseptic surgical technique. Catheters are directed into the maternal aorta which then allows specific cannulization of the right or left uterine artery. Radiopaque dye is then infused and the arterial supplied to the cotyledons identified under various levels of magnification via radiographs or cinematography. In some instances, an attempt is made to embolize segments of the placenta with the use of liquid silicone.