Inulin was administered to eight volunteer patients at term gestation over a period of 3 hours before cesarean section. Inulin concentrations were repeatedly measured in maternal plasma, in fetal plasma, and in amniotic fluid at the time of delivery. Total inulin uptake of the conceptus was taken to be the sum of the inulin in the amniotic fluid and in the newborn infant. Amniotic fluid volumes were measured by ultrasound examination, and the distribution volume of inulin in the neonate was assumed to be 180 ml/kg on the basis of animal experiments. The mean permeability was 0.15 microliter/(s.g) placenta. This value and the previously measured permeability for cyanocobalamin delimit a range of molecular weights from 1350 to 5200 daltons. In this range permeability to lipid-insoluble molecules is roughly proportional to the coefficients of free diffusion in water without further discrimination of molecular size by the placental barrier.