Control measurements of uterine blood flow (N = 110) or umbilical blood flow (N = 84) and the oxygen content difference across the circulation in question were analyzed to assess the effect of flow rate, measured by the diffusion equilibrium technique, on the oxygen uptake of either the uteroplacental-fetal tissue mass or the fetus alone. Although the rate of uterine blood flow varied widely (110 to 418 ml. per kilogram per minute, the rate of uterine oxygen uptake did not change significantly at low or high rates of flow. Likewise, the rate of umbilical blood flow varied widely (154 to 444 ml. per kilogram per minute) without a significant change in fetal oxygen uptake at the extremes of flow. In both circulations this was due to the existence of a reciprocal relationship between flow rate and deltaO2.