A study of the physiochemical characteristics of placental transfer, using a biophysiochemical model of maternal-fetal circulatory and metabolic relations simulating the composition and transfer of respiratory gases and other elements across the placental membrane. The construction of a biophysicochemical model of certain maternal-fetal circulatory and metabolic relations for an extra-uterine study of the transfer of respiratory gases and other elements across the placental membrane. The model is analyzed by a mathematical method for the minimization of a chemical free-energy function subject to constraints relating to mass, charge, and phase transfer. The model is then applied to the representation of the exchanges of respiratory gases occurring between the venous and arterial sides of the total air-blood system. The model indicates a greater acidity for the fetal than for the maternal erythrocite intracellular medium. This feature, combined with other aspects of the results, may explain the lower oxygen saturation of fetal hemoglobin in utero, and also suggests that the fetal oxygen environment is not so inimical or stressful to the fetus as previously hypothesized.