In order to investigate whether the oral ingestion of collagen peptide affects the extracellular matrix of tendon, two doses (0.2 g/kg and 1.0 g/kg body weight) were orally administered daily for 56 d to a rabbit, and both the size of collagen fibrils and the amount of glycosaminoglycans in the Achilles tendon were measured in comparison with those in a rabbit fed with a control protein, lactalbumin, or water alone. Ingestion of collagen peptide or lactalbumin induced a significant increase in collagen fibril diameter and a decrease in fibril density except for a high dose of lactalbumin compared with the water control. A histogram pattern of fibril diameter in a high dose of collagen peptide showed a peak at 160-180 nm, which was not observed in other groups. However the percentage of diameters over 200 nm was the lowest in this group but highest in the low-dose group of collagen peptide. The mean fibril diameter and mass average diameter of a high dose of collagen peptide were significantly smaller than those in a low dose. The amount of dermatan sulphate increased in the high-dose groups, while the amount of hyaluronic acid decreased in rabbits fed with collagen peptide or lactalbumin at either dose. These results suggest that the ingestion of collagen peptide affects the size of collagen fibrils and composition of glycosaminoglycans in the Achilles tendon and thus may improve the mechanical properties of the Achilles tendon.