The normal relationship between fetal and maternal plasma of total osmotic pressures and concentrations of sodium and potassium were determined in women with uncomplicated pregnancies at full term. The total osmotic pressure of fetal plasma was found to be 3.6 mOsm/kg of water higher than the maternal, probably not representing a physiologically significant difference. No significant difference in sodium concentrations of plasma was found. The potassium concentration in the fetal plasma was 0.7 meq/l higher than in the maternal plasma. The administration of 5% glucose or 20% mannitol intravenously to pregnant women prior to delivery was shown to have a prompt effect upon the total osmotic pressure and concentration of sodium and total protein of fetal plasma.