In the current study, the pathogenesis of proximal tubular cyst formation was studied in an animal model of polycystic kidney disease, the CPK mouse. The specific roles of (a) sodium-potassium adenosine triphosphatase (Na−K ATPase) activity, determined by an enzyme-linked kinetic microassay, (b) proximal tubular epithelial hyperplasia, determined by calculation of mitotic indices, and (c) altered proximal tubular basal lamina formation, determined by immunohistological localization of basal lamina glycoproteins, were investigated at progressive developmental stages of CPK proximal tubular cyst formation. Increases in renal Na−K ATPase were present at the earliest fetal stages of proximal tubular cyst formation, and subsequently paralleled the course of proximal tubular cyst progression. Proximal tubular epithelial hyperplasia, although not present at the earliest stages of cyst formation, was a consistent feature of progressive proximal tubular cystic enlargement. Abnormalities in basal lamina glycoprotein expression were not present at any stage of proximal tubular cyst development. We conclude that increased Na−K ATPase and tubular epithelial hyperplasia are significant features of proximal tubular cyst formation in the CPK mouse.