Pairs of glomerular and tubular basement membrane preparations were obtained from kidneys of individuals of premature age up to 80 years. The purity of the preparations was established with light and electron microscopy and by estimating the total phosphorus content. The amino acid and carbohydrate composition was determined and statistically evaluated for 33 to 38 preparations divided over five age groups. Comparison of glomerular and tubular basement membranes from the same kidneys showed that regardless of age glomerular basement membranes contain more 3-hydroxyproline and more of the heteropolysaccharide constituents neuraminic acids and mannose. These differences cannot be ascribed solely to the presence of a minor amount of interstitial collagen. The chemical composition of the two types of basement membranes changes with age. For the first few years after birth the contents of glycine and total imino acids increase and those of collagen-nonspecific amino acids decrease, whereas constituents of the heteropolysaccharide units do not change markedly. These results suggest that proportions of collagenous and noncollagenous peptide moieties gradually change with age in both glomerular and tubular basement membranes. In addition, the extent of hydroxylation of proline and lysine increases significantly with age, reaching an adult level for glomerular basement membranes after 4 to 7 months of age and for tubular basement membranes during late childhood. The contents of glucose and galactose rise with age to an extent comparable with that of hydroxylysine. The age-related changes in basement membrane composition may influence functional properties of these extracellular renal structures.