Fifteen pregnant ewes were studied to determine what fractions of the increases in maternal cardiac output and oxygen consumption found in pregnancy were due to the concomitant increases in blood flow to and oxygen consumption of the uterus and its tissue contents. Maternal cardiac output rose at term to 6.65 +/- 0.47 L. per minute, a value 40 per cent above the postpartum level. Uterine blood flow, which had increased at term to 1.62 +/- 0.15 L. per minute, represented 24 per cent of maternal cardiac output above postpartum levels. Maternal oxygen consumption at term was 259 +/- 11 c.c. per minute, a value 33 per cent above the postpartum level. Uterine oxygen consumption, which had increased at term to 57 +/- 4 c.c. per minute, represented 22 per cent of maternal oxygen consumption and accounted for 87 per cent of the increase in maternal cardiac output above postpartum levels. These data indicate that in the ewe most of the pregnancy-associated increases in maternal cardiac output and oxygen consumption are due to the demands of the uterus and its tissue contents.