S U M MARY Serum concentrations of ,B-alanine and L-histidine are compared in five normal adults after ingestion of the dipeptide carnosine (/-alanyl-L-histidine) and after equivalent amounts of the constituent free amino acids. The results indicate that absorption is significantly more rapid after the ingestion ofthe amino acids than after the dipeptide. The use of the test in a case of Hartnup disease suggests that carnosine is taken up by intestinal cells as the dipeptide, but subsequent hydrolysis and delivery of the constituent amino acids to the portal blood is a slower process than transport of the free amino acids themselves.