P revious studies have shown that bile salt concentrations in human blood taken from the placenta at birth of term infants are in the range found in adults. A 125I-radioimmunoassay procedure and capillary gas liquid chromatography-mass spectrometry have been used in this investigation to measure serum bile salt concentrations in premature and normal term infants. It was found that the serum bile salt concentration in samples taken at birth in premature infants were also similar to that of adults. In the week after birth the serum bile salt concentration rose four- to sevenfold in each of the infant groups. The increase was independent of gestational age and the "health" of the child. A similar increase was observed in term infants. Thus, hypercholemia is physiologic in newborn infants. In conjunction with other abnormalities of the enterohepatic circulation of bile salts there are profound implications in the newborn for the metabolism and excretion of those endogenous and exogenous substances that are dependent on the secretion of bile salt by the liver. In addition, speculations concerning the role of parenteral nutrition in the induction of cholestasis in premature infants should be made with caution.