1. In pregnant rabbits near term, cardiac output and its distribution were measured by injection of isotope-labelled microspheres into the left ventricle. Hypoxaemia (mean P(a,O2) 35 mm Hg) caused a reduction in placental and renal flows, while myometrial flow was unchanged and coronary and cerebral blood flows increased.2. In the lateral (ovarian) end of the uterine horn foetal and placental weight and absolute maternal placental blood flow were greater than in intermediate or medial positions. Placental flow (per 100 g placental tissue) was most in the most lateral placenta and decreased in the intermediate positions, especially when there were many placentas in the same horn.3. Abortions and runts (more than 2 S.D. below the mean weight for their age) occurred most often in the medial end of the uterine horn, while oversize foetuses (more than 2 S.D. above the mean weight) usually occupied the most lateral position.4. Ovarian blood flow (460 ml./100 g. min) was relatively large (e.g. compared with renal flow, 275 ml./100 g.min).