Intercalated cells play a major role in proton and bicarbonate secretion in the collecting duct of kidney. A third type of intercalated cell (non A-non B cell), besides type A and B intercalated cells, and a bipolar cell are known to exist in the kidneys of the rat or the mouse. The third type cell has H(+)-ATPase in the apical membrane like the type A intercalated cell, but has no Cl(-)-HCO(3)- exchanger (AE1) on the basolateral membrane. The bipolar cell was shown to express H(+)-ATPase on both the apical and basolateral membranes. The functions of these cells, however, are not determined yet. This study was intended to know the immunohistochemical changes of the intercalated cell subtypes in the acute respiratory acidosis and alkalosis. After midline tracheostomy, respiratory acidosis and alkalosis were induced and maintained for 4 hours in the Sprague-Dawley rats (450~500 g) using a Rodent Ventilator. The kidneys were preserved for immunohistochemical studies by in vivo perfusion fixation with periodate-lysine-paraformaldehyde solution through the abdominal aorta. To identify the subtypes of intercalated cells and the tubule segments in which they are located, a triple immunolabeling procedure was used. Distal convoluted tubule cells and principal cells in the collecting duct were identified using antibody to thiazide sensitive Na(+)Cl(-) cotransporter and antibody to aquaporin-2, respectively. Antibodies to H(+)-ATPase and AE1 were used to identify subpopulation of intercalated cells. Type A cells were activated in respiratory acidosis with enhanced AE1 activity on the basolateral membrane and H(+)-ATPase reactivity moved to the apical membrane, whereas inactivated in respiratory alkalosis with decreased AE1 reactivity and H(+)-ATPase reactivity moved to the supranuclear cytoplasm. The change in reactivity of type A cells in respiratory acidosis or alkalosis was shown to differ depending on the tubular segments: most of the intercalated cells were activated in the outer medullary collecting duct while only a portion of the type A cells activated in the distal convoluted tubule, connecting tubule and cortical collecting duct. No changes were observed in type B cells in respiratory acidosis and alkalosis. In non A-non B cell which was increased in size in respiratory acidosis, H(+)-ATPase reactivity was seen on the apical membrane in respiratory acidosis, while seen in the supranuclear cytoplasm in respiratory alkalosis. These findings indicated that the renal compensation for respiratory acid-base imbalance was mediated mainly by type A cells rather than by type B or non A-non B cells. Among type A cells, more of those of outer medullary collec-ting duct were thought to be recruited compared with those of the cortical collecting duct and connecting tubule.