1. In anaesthetized rabbits 27-29 days pregnant (term is 31 days) uterine blood flow was measured in an extracorporeal loop between a carotid and the central end of an external iliac artery, the descending aorta and its other branches being tied. Mean flow was 45 +/- 2 ml. min(-1), or 105 +/- 4 ml. min(-1) per kg uterus and contents in thirty-four litters.2. The partition of uterine blood flow to placentas ( approximately 60%), myometrium ( approximately 27%) and vagina ( approximately 13%) as estimated by injection of isotopelabelled microspheres of 35 or 50 mu diameter, was similar in intact rabbits and in those with an extracorporeal loop.3. Maternal blood flow was greater to the placentas of larger foetuses both absolutely and per unit placental weight. Flow to the placentas of dead foetuses was much less than to those of live foetuses, but flow to the adjacent myometrium was not decreased.4. Maternal hypoxaemia (P(a, O2) approximately 40 mm Hg) caused vasoconstriction in the uterine vascular bed after 3-4 min. The distribution of blood flow within the uterus was altered, the placental fraction falling to 32%. This was unaffected by carotid denervation.5. These observations suggest that there are large differences in the physiological control of maternal blood flow to the placenta, myometrium and vagina.