Almost all substances that are passively transferred fall into two classes. Their transfer is either purely "flow limited" or purely "diffusion limited," with few in between. Diffusion limited transfer (most lipid insoluble materials) is determined by the diffusion permeability of the interhemal membrane. In the epitheliochorial placenta, this permeability declines precipitously with increasing molecular weight and with the presence of electric charge. In the various hemochorial placentas, molecular weight is not discriminated, except to the extent that it affects the coefficient of free diffusion in water and electric charge is of little or no consequence. Flow limited transfer applies to those substances (lipid soluble) whose concentrations in maternal and fetal blood equilibrate in a single pass through the placenta. Transfer of oxygen is flow limited or nearly so. The effectiveness of flow limited transfer depends on vascular geometry. The "counter current" labyrinthine placentas of the rabbit and the guinea pig are highly efficient, whereas the villous placentas of sheep, monkey and man, which appear to allow end venous equilibration, are much less so. The price of efficient flow limited transfer may be less reserve when one of the blood flows declines. Maternofetal water flow per unit surface area of the placenta may be much greater in the embryo than in the fetus. If so, ultrafiltration may play more than a negligible role in the embryo in the transfer of lipid insoluble solutes and species differences may be less pronounced than in the fetus. A relation between placental transfer regime and evolutionary pressure is not yet apparent.