The absorption and excretion of riboflavin, a vitamin which is absorbed in adults by a saturable transport process in the proximal small intestine, was investigated in healthy, 5-day-old infants. After oral administration of a saturation dose of the vitamin (150 mg/sq m), the urinary recoveries of riboflavin in the neonates were similar (about 7% of the dose) to those obtained previously in 3-to 6-month-old infants. However, the maximum excretion rate (per unit of body surface area) of riboflavin was much lower and the duration of maximum excretion rates was much longer in the neonates than in older infants. Data obtained after parenteral administration of riboflavin show that the low maximum excretion rate was not due to limited renal capacity and that the longer duration of excretion was due to prolonged absorption of the vitamin from the intestinal tract.