Polycystic renal disease was induced in rats by feeding 2-amino-4,5-diphenylthiazole. Tubular (TBM) and glomerular basement membranes (GBM) were purified and analyzed for possible structural changes that may be a factor in the development of the tubular dilations and cysts. Changes in the relative quantities of TBM polypeptides were detected by sodium dodecylsulfate polyacrylamide gel electrophoresis. An overall increase in the concentration of high molecular weight components and a decrease in concentration of those of low molecular weight components were observed. Changes which were particularly notable included a twofold increase in a component of Mr = 380,000 and a decrease in one of Mr = 55,000 as analyzed without reduction of disulfide bonds. With reduction of disulfide bonds, the Mr = 380,000 component dissociates, whereas the Mr = 55,000 polypeptide does not, and polypeptides of Mr = 245,000 and 145,000 are observed to does not, and polypeptides of Mr = 245,000 and 145,000 are observed to increase about twofold in concentration (approximate molecular weights were determined using globular protein standards). These changes take place most rapidly from 4 to 8 weeks of drug administration and remain relatively constant between 8 and 16 weeks. If feeding of the drug is discontinued, the distribution of TBM polypeptides returns to normal. These results indicate that tubular basement membrane from animals with 2-amino-4,5-diphenylthiazole-induced polycystic renal disease is abnormal, and this should be considered as a possible contributing factor in the formation of cysts.