Renal tubular and glomerular basement membranes were isolated from rabbit, rat and mouse kidneys. Methods were developed to obtain the basement membrane from limited numbers of animals. Rabbit and mouse tubular basement membrane as well as rabbit and rat glomerular basement membrane were obtained from single animals in quantities sufficient for electrophoresis and chemical analysis. The chemical composition of all the basement membranes were compared and sodium dodecylsulfate-polyacrylamide gel electrophoresis was performed on rabbit tubular and glomerular basement membranes in order to compare their polypeptide composition. The chemical composition of the tubular basement membranes were similar, each species having nearly equal concentrations of glycine, hydroxyproline and hydroxylysine, respectively. Glomerular basement membranes were also similar to each other and close to tubular basement membranes in chemical composition. Within each species, the glycine, hydroxyproline and hydroxylysine values for tubular basement membrane were about 10% higher than for glomerular basement membrane. The polypeptide composition of a reduced, sodium dodecylsulfate-soluble fraction of the rabbit basement membranes appeared to be alike with two exceptions. A prominent band of Mr = 160,000 seen in gels of tubular basement membrane was present as a lightly staining band in glomerular basement membrane samples and a prominent band of Mr = 140,000 in gels of glomerular basement membrane was seen as a light band in samples of tubular basement membrane.