The effect of acetate, at physiologic concentrations, on sodium and water absorption in the temporarily isolated colon of the conscious pig was studied with isotonic perfusion solutions buffered at pH 6.4 or 7.4, in which either chloride or acetate was present as the major anion. Sodium and water absorption was greater from the acetate solution when the pH was low; however, acetate reduced sodium and water absorption below that observed with the chloride solution when the pH was high. There were no significant differences observed in sodium or water absorption from the chloride solution at either pH. We conclude that the concentration of undissociated acetate influences sodium and water absorption in the colon of the pig. Such an effect may have important consequences on the reserve absorptive capacity of the colon in certain diarrheal diseases.