BOTH the pediatrician and the obstetrician need to know what influences various events during fetal development have on the performance of the infant after birth. It would be desirable to have more of the studies in this general area appear in pediatric journals to emphasize its importance to pediatricians. An example is the matter of the forces which govern the movement of water across the placenta. In general, this problem has been studied by determining the concentration of total solute, or what is more commonly termed the total osmotic pressure, of the plasma on the two sides of the placental barrier.