The effects of acute metabolic alkalosis and acidosis on intestinal electrolyte transport were studied in adult Sprague-Dawley rats. Animals were made alkalotic or acidotic by gavage feeding of 1 M solutions of NaCl (pH = 7.42), NaHCO3 (pH = 7.52), NH4Cl (pH = 7.18), or 0.75 M (NH4)2SO4 (pH = 7.21). After 1-3 h, animals were anesthetized and prepared for in vivo perfusion of the jejunum, ileum, and colon. The jejunum exhibited increased net potassium absorption in alkalosis and decreased potassium absorption in acidosis. In the ileum, net sodium absorption and potassium secretion were decreased, and bicarbonate secretion was increased in alkalosis, and opposite effects were observed in acidosis. The ileal lumen minus blood gradient for PCO2 (an index of hydrogen ion secretion) was greater in acidotic than in alkalotic animals. The levels of ileal sodium, bicarbonate, and potassium transport and the PCO2 gradient correlated well with the plasma pH and bicarbonate concentration in individual animals. In the colon, net bicarbonate secretion and chloride absorption increased and potassium secretion decreased in alkalosis, and opposite effects were observed in acidosis. The colonic lumen minus blood PCO2 gradient was not affected by acid-base balance. Colonic bicarbonate transport correlated with the plasma chloride concentration as well as with the plasma pH. The acid-base disorders had no effect on transmural potential difference. These results suggest that acute metabolic alkalosis and acidosis alter sodium and hydrogen ion transport in the ileum and chloride and bicarbonate transport in the colon.