The oxygenation and metabolism in appropriate (AGA) and small for gestational age (SGA) fetuses has been investigated by cordocentesis. The umbilical venous and arterial pO2 and pH decrease with gestational age while pCO2 increases and blood lactate concentration does not change. The mean umbilical venous blood glucose concentration is higher than in the umbilical artery indicating that there is fetal glucose uptake from the placenta. Similarly, the maternal glucose concentration is higher than the fetal and the levels in the two compartments are significantly correlated. The plasma insulin concentration increases exponentially with gestation reflecting the progressive maturation of the fetal pancreas. The fetal plasma cortisol does not change but the fetal plasma ACTH increases with gestation. Fetal plasma triglyceride concentration decreases exponentially with gestation and this is likely to be the result of increased utilization by the fetus for deposition into adipose tissue. There is a high correlation between fetal and maternal levels for individual amino acids and the concentration in the fetus is higher than in the mother, supporting the active transport of amino acids by the placenta. Some SGA fetuses are compromised by hypoxemia, hypercapnia, hyperlacticemia and acidosis, are starved of glucose and amino acids, and are hypertriglyceridemic. Furthermore, some of these fetuses are hypoinsulinemic and the degree of hypoinsulinemia is disproportional to the degree of hypoglycemia suggesting pancreatic dysfunction. In SGA fetuses the plasma cortisol is increased and the plasma ACTH decreased. Knowledge of human fetal oxygenation and metabolism may help in deciding the optional timing of delivery but may also constitute a basis for future fetal therapy in the form of oxygen and nutrient supplementation.