Tubular function studies have shown that nephrectomy leads to a sharp increase in potassium secretion by initial collecting tubules. The aim of this study was to examine the relationship between potassium transport and the cellular morphology of the distal nephron. Clearance and ultrastructural studies were conducted in rats 10 days after 75% nephrectomy. Fractional urinary potassium excretion increased from 23.4 +/- 3.8% in controls to 63.9 +/- 10.1% in nephrectomized animals. Stereological analysis of principal cells in the initial collecting tubule revealed that following nephrectomy the surface density of the basolateral membrane increased from 2.32 +/- 0.12 to 4.11 +/- 0.20 micron2/micron3. While the basolateral membrane surface density of intercalated cells in this segment was not affected by nephrectomy, the luminal membrane surface density increased from 0.29 +/- 0.03 to 0.54 +/- 0.08 micron2/micron3. Ultrastructural changes were not observed in cells of the distal convoluted tubule. These observations demonstrate that cell type specific changes in membrane surface density accompany the adaptive increase in ion transport that occurs with a reduction in renal mass. The enhanced potassium excretion associated with nephrectomy may be partly mediated by a specific increase in the basolateral membrane of principal cells. The increase in luminal membrane of intercalated cells may be related to the increase in acid transport per surviving nephron following nephrectomy.