In the near-term pregnant ewe, acute treadmill exercise produced significant changes in maternal and fetal parameters only with the onset of maternal exhaustion which was unrelated to the degree or duration of maternal exercise. With exercise to exhaustion the rate of both uterine and umbilical blood flow fell but uterine and umbilical oxygen uptakes were unchanged. Although maternal lacticemia developed and fetal PO2 fell no excess lactate was produced by the uteroplacental tissues or fetus. The increase in maternal heart rate during exercise was directly related to the decrease in uterine blood flow.