The proportion of glomerular basement membrane (GBM) solubilised following pepsin treatment increased with age when the digestion was carried out at 4°C, but remained constant at 50% of the membrane when the digestion was carried out at 10°C irrespective of rat age. Amino-acid analysis of the solubilised material indicated that it became more collagen-like with age, as judged by the increase in glycine, hydroxyproline and hydroxylysine content, if the digestion was carried out at 10 °C. Polyacrylamide gel electrophoresis of pepsin-solubilised material demonstrated that components with apparent molecular weights of 180,000, 160,000 and 130,000 increased in intensity with age. Components with apparent molecular weights < 85,000 were only present in pepsin-soluble material prepared from GBM isolated from young rats. The intensity of the bands in the pro-α-region varied with age and some procollagen-like material was not solubilised by pepsin treatment. These studies confirm that aging of rat GBM is characterised by an increase in the proportion of the collagenous components present in rat