Acute respiratory alkalosis and acidosis alter rat ileal and colonic but not jejunal electrolyte transport. To examine the role of altered intracellular pH, pHi and HCO3 concentration, (HCO3)i, pHi was measured in mucosa scraped from the jejunum, ileum and colon of anesthetized, mechanically ventilated Sprague-Dawley rats. During states of respiratory alkalosis (PCO2 [CO2 partial pressure] 24.9 .+-. 0.8 mm Hg, pH 7.586 .+-. 0.014), respiratory acidosis (PCO2 67.8 .+-. 1.2 mm Hg, pH 7.228 .+-. 0.007) and normocapnia (PCO2 41.1 .+-. 0.7 mm Hg, pH 7.401 .+-. 0.006), pHi was measured by determining the distribution of 5,5-dimethyl[2-14C]oxazolidine-2,4-dione, using [3H]inulin as a marker of extracellular space. (HCO3)i was calculated using portal vein PCO2. In the ileum, the pHi of 6.901 .+-. 0.029 was similar in alkalosis [(HCO3)i 5.4 .+-. 0.3 mM], acidosis [(HCO3)i 12.4 .+-. 0.6 mM] and normocapnia [(HCO3)i 8.6 .+-. 0.8 mM). In both the jejunum and colon, pHi was increased in alkalosis [pHi 6.998 .+-. 0.038, (HCO3)i 6.7 .+-. 0.6 mM] and decreased in acidosis [pHi, 6.789 .+-. 0.024, (HCO3)i 10.4 .+-. 0.6 mM] as compared with normocapnia [pHi 6.915 .+-. 0.026, (HCO3)i 8.9 .+-. 0.7 mM] (colon data given). Net electrolyte transport measured by in vivo perfusion revealed that ileal and colonic, but not jejunal, net Na and Cl absorption was decreased during alkalosis and increased during acidosis. During respiratory acidosis and alkalosis, pHi is apparently maintained in a qualitatively similar way in the jejunum, ileum and colon with quantitatively greater or lesser changes in (HCO3)i. In addition, the difference between the jejunal, ileal and colonic electrolyte transport responses suggests the presence of (pH/PCO2/HCO3)i-sensitive transport mechanisms in the rat ileum and colon but not in the jejunum.