Net transport of electrolytes and water, unidirectional Na+ and Cl- fluxes, and mucosal permeability were examined in isolated colonic loops of control pigs and pigs infected with Treponema hyodysenteriae. Results indicated that all net ion and water absorption from the colon was abolished in infected pigs. This abolition of net absorption was almost exclusively the result of a decrease in the lumen-to-blood fluxes of Na+ and Cl-; the blood to lumen fluxes and mucosal permeability were essentially unaffected. The results indicated that the diarrhea observed in these pigs was a consequence of colonic absorptive failure alone. Neither an active nor a passive secretory process could explain the fluid losses under the conditions of these experiments.