Fetal blood volume was measured in pregnant ewes bearing chronically cannulated fetuses in the last third of gestation. The increase in fetal blood volume secondary to growth was established from serial measurements in four control animals. The effect of maternal dehydration was assessed by measuring the change in fetal blood volume during twelve episodes of maternal water deprivation in another six ewes. Fetal blood volume increased during each episode of maternal dehydration. In five experiments the rate of increase was greater than controls and in seven experiments the rate of increase was less than controls. Thus, fetal hypovolaemia does not occur during maternal dehydration in the ovine fetus in the last third of gestation.