The outer medullary collecting duct which is composed of both principal and intercalated cells is involved in hydrogen ion secretion. In the turtle urinary bladder stimulation of hydrogen ion secretion is associated with ultrastructural changes in the mitochondria-rich cells, suggesting that membrane and possibly a proton pump are being transferred from apical tubulovesicular structures and inserted into the apical plasma membrane. Since the intercalated cells resemble the mitochondria-rich cells, this study was initiated to determine whether or not similar changes occur in the outer medullary collecting duct during chronic metabolic acidosis. Rats received ammonium chloride in their drinking water for 15 days and as a daily gavage for 3 days before sacrifice. Control rats received regular tap water. After collection of physiologic data the kidneys were fixed by in vivo perfusion with glutaraldehyde and processed for electron microscopy. No changes were observed in the principal cells. Morphometric analyses of the intercalated cells in both the outer and inner stripe revealed a significant increase in the surface density of the apical plasma membrane concomitant with a striking depletion of the tubulovesicular structures in the apical plasma region of the cell with chronic metabolic acidosis. These findings suggest that in response to chronic metabolic acidosis membrane, possibly containing a proton pump, is transported from the tubulovesicular membrane compartment to the apical plasma membrane of the intercalated cell.