The chemical composition of rat tail tendon was determined in rats in different ages from 1 month to 24 months. Total protein and calcium chloride extractable (CTC extract) protein decreases steadily with age and particularly sharply between the 1st and 3rd month of age. Extractable hydroxyproline (soluble collagen) decreases very strongly between the 1st and 3rd month, remains then constant and decreases again between the 17th and 24th month of age. Polymeric collagen (TCA extract) increases sharply between the 1st and 3rd month of age and shows then relatively little changes with a slight decrease only from the 3rd month to the 24th month. Structural glycoproteins (urea-extract) decrease from the 1st to the 10th month and remain then relatively constant. Their relative ratio to total protein of the tendon does not change with age. Total hexose decreases from the 1st to the 3rd month of age as does hexose and hexosamine in a 0.9% sodium chloride extracted tendon residue. The ratio of galactose, glucose, mannose and xylose or fucose changes also between the 1st to the 3rd month: glucose and mannose decrease with respect to galactose and remain then constant. Xylose or fucose continue to decrease up to the 24th month. Lysine derivatives determined by high voltage electrophoresis do not show this age change. Lysine, hydroxylysine-glycosides, remain in a constant ratio of 1.0 to 0.3 to 0.06. Only trace amounts of hydroxylysine-galactoside are present in rat tail tendon collagen, nearly all of the hydroxylysine linked hexoses are present as glucosido-galactoside. Two unknown spots observed on high voltage electrophoresis of alkaline hydrolysates of rat tail tendon collagen correspond probably to cross-link amino acids and show also a constant ratio to lysine. These results indicate that certain components of rat tail tendon change with age, others do not. Strongly decreasing components are freshly synthesised collagen and some glycoproteins. No age change was observed in the ratio of hydroxylysine linked glycosides as well as in the ratio of structural glycoproteins to total proteins. Some of these results are similar to those which were obtained on chick embryo corneas [15]. There also we observed a decrease of structural glycoproteins simultaneously with an increase of the collagen content of the corneas.