BACKGROUND: Copper homeostasis involves a high degree of regulation in which changes in absorption and biliary excretion are the main mechanisms. Whether neonates and small infants can make these changes efficiently is unknown. OBJECTIVE: We evaluated the effect of age and copper intake on copper absorption in infants during the first 3 mo of life. DESIGN: Thirty-nine healthy infants (19 infants aged 1 mo and 20 infants aged 3 mo) were selected. One-half of the subjects were randomly assigned to receive oral supplementation of 80 mg Cu (as copper sulfate). kg body wt(-1). d(-1) for 15 d. At the end of the trial, copper absorption was measured by using orally administered (65)Cu as a tracer and fecal monitoring of recovered (65)Cu. RESULTS: Mean (+/- SD) copper absorption at 1 mo of age was 83.6 +/- 5.8% and 74.8 +/- 9.1% for the unsupplemented and supplemented infants, respectively. The corresponding figures at 3 mo of age were 77.6 +/- 15.2% and 77.7 +/- 11.3%. A two-way analysis of variance showed that age, copper supplementation, and the interaction between age and copper supplementation did not have a significant effect on copper absorption. There was an inverse correlation between total fecal copper and the percentage of (65)Cu absorption (r = -0.50, P < 0.003). CONCLUSION: Copper absorption in young infants is high but does not respond to copper intake within the range tested.