Immaturity of hepatic excretory function resulting in a period of "physiologic cholestasis" may occur during early life. Serum bile acids should accurately reflect maturation of the enterohepatic circulation; we therefore determined serum concentrations of the primary bile acids in normal infants to define age-related changes. There was a striking rise in serum cholylglycine and conjugates of chenodeoxycholate during the first few days of life over levels detected in cord sera; the values attained were significantly greater than maximal postprandial concentrations found in children over 1 yr of age (p less than 0.01). There was a gradual decline in bile acid concentration; however cholylglycine remained higher than the postprandial values of older children until 4 mo and chenodeoxycholate until 6 mo of age. In 12 infants a liquid feeding stimulated a greater maximal postprandial cholylglycine concentration and integrated area under the meal curve than that achieved in children (p less than 0.01). There was no difference in the postprandial response in chenodeoxycholate in these infants compared with the older subjects. We conclude that serum bile acids are elevated in normal infants and that the subsequent decline to levels of the child and adult demonstrates the evolving maturation of liver function during infancy.