The distribution of the cardiac output and the maternal placental blood flow has been measured in guinea-pigs at days 49-51 of pregnancy using radioactively labelled microspheres. In some instances uterine blood flow was reduced chronically by ligating one uterine artery at day 30 of pregnancy. Between 3 and 4% of the cardiac output passed normally to placenta, and this could be reduced to less than 2% after uterine artery ligation. The result of the ligation was to reduce fetal and placental weight by up to 70%. Fetal and placental weight showed a close linear correlation in controls and in pregnancies with uterine artery ligation. However, when placental size was reduced below 60% of control, fetal weight was less affected by a reduction in placental weights than normal. Placental blood flow and placental size exhibited a close linear relationship over the whole range of values, but there was limiting placental weight which approached 1.3 g as placental blood flow approached zero. Thus a reduced placental size, particularly below about 50%, was associated with a proportionately greater reduction in maternal placental blood flow. Maternal placental blood flow or the percentage of maternal cardiac output to the placenta and fetal weight also showed a well-correlated linear relationship. However, when placental blood flow was below about 50% of control values further reduction had a less than normal effect upon fetal growth. Small fetuses were hypoglycaemic and hypoinsulinaemic and the degree of each was dependent upon the extent of the reduction in fetal weight and in maternal placental blood flow. In fetuses that were below about 40% of normal size, and in which placental blood was below about 30% of control, fetal weight was less sensitive to falls in blood glucose, which in turn was more sensitive than normal to a fall in maternal placental blood flow. The results indicate that over the range of 50-100% of normal fetal growth, maternal placental blood flow and probably nutrient supply to the fetus vary in parallel. Hence over this range fetal and placental growth rates are determined in part by placental blood flow. At placental blood flow rates and fetal growth rates below 40% of normal fetal growth is less dependent upon placental blood flow than usual, presumably because of a reduced dependence upon glucose metabolism for growth. This would appear to be essential, since as maternal placental blood flow is reduced to low values the placenta has to utilize an increasing proportion of the available glucose.