To delineate changes in the development and regression of renal cystic disease, sequential studies were done in rats fed 2-amino-4,5-diphenyl thiazole (DPT). Kidneys were perfusion fixed with either aldehyde fixative alone or with the addition of ruthenium red (RR). Tissues were processed for light and electron microscopy. Light microscopy sections were stained with alcian blue and eosin and hematoxylin. Initially, cellular proliferation and, later, cystic transformation of collecting tubules were observed. The structural changes in tubular cells preceded alterations in the basement membrane and consisted of an increase in smooth and rough endoplasmic reticulum and free polyribosomes, prominence of Golgi complexes, and an increased number of lysosomes. These findings are suggestive of changes in the biosynthetic, secretory, and degradative mechanisms of the cell. With time, the tubular basement membranes became progressively thicker and laminated with concomitant loss of alcian-blue- and RR-staining. When DPT-treated animals with renal cystic disease were placed on a normal diet, tubular cell and basement membrane morphology and alcian-blue- and RR-staining returned to normal and cystic changes regressed. These findings are compatible with altered synthesis and degradation of tubular basement membranes in this model of cystic disease.