Investigation of the various factors which control the placental circulation has necessarily been limited owing to the experimental difficulties involved. It is suggested that the following technic may offer a line of approach to some of the problems involved.Methods. The experimental animals used were cats or bitches which were in an advanced stage of pregnancy. Rabbits were found to be unsuitable owing to the friability of the placental vessels. Anesthesia was induced with open ether, and maintained with chloralose given intravenously in a dose of 100 mg/kilo body weight. Both vagi were cut, and the carotid blood pressure recorded by means of a mercury manometer. The abdomen was opened with as short an incision as was compatible with adequate access, and a part of the uterus containing one foetus brought up into the wound. A small cork platform approximately 5 cm square was approximated to the section of uterus exposed, and the uterine peritoneum loosely stitched to the two nearest corners of the platfo...