The composition of the glomerular basement membrane (GBM) was studied in three patients with the congenital nephrotic syndrome of the Finnish type (CNF). A decrease was found in the relative amounts of amino acids characteristic of collagen, i.e. 3-hydroxyproline, 4-hydroxyproline, hydroxylysine and glycine. The decrease of hydroxyproline was also apparent in the ratio of 4-hydroxyproline to proline and in direct assays of 4-hydroxyproline in whole glomeruli. The ratio of 3-hydroxyproline to 4-hydroxyproline was decreased by about 40%. Slight increases were found in the amounts of some amino acids. No significant change was found in the glucose content of the GBM whereas the galactose content was slightly decreased. The results suggest a decrease in the relative amount of the collagen component in the GBM of CNF patients, but a decrease in the relative amount of 3-hydroxyproline indicates that other changes also exist.