Bile salt metabolism was examined in 12 infants (ages 7 weeks to 10 months) who had recovered from protracted infantile diarrhea. Cholic acid kinetics were measured with the isotopic dilution technique. Cholate pool size was 938 +/- 89 mg/m2 (means +/- S.E.), synthetic rate was 478 +/- 52 mg/m2/day, and the fractional turnover rate was 0.537 +/- 0.060 day -1. Estimated chenodeoxycholate pool size was 607 +/- 103 mg/m2. similar cholic acid pool sizes were previously observed in older children; however, the synthetic and turnover rates were significantly lower, p less than 0.05 and 0.025, respectively. A significant inverse relationship between age and fractional turnover rate (r = -0.577, p less than 0.05) was observed in the first 10 months of life. Fasting serum cholylglycine measured by radioimmunoassay was significantly higher (p less than 0.01) in infants than in children. Peak postprandial concentrations were higher in infants than in older children. Accelerated hepatic bile salt synthesis rapidly increases the size of small pools at birth. Intraluminal bile salt concentrations concurrently increase, thereby normalizing fat solubilzation. Slow maturation of intestinal transport mechanisms may result in normal cycling of bile salts by 3 to 7 months of age. Persistent immaturity of hepatic uptake and/or excretion of bile salts leads to intrahepatic retention of a portion of the pool, with resultant regurgitation into the serum during the first months of life.