The mechanism of the enhancement of the intestinal absorption of sulfaguanidine by bile salts, sodium taurocholate and sodium glycocholate, together with the poor absorbability of the drug itself was investigated, using the in situ single loop preparations. It was found that the cause of the poor absorbability of sulfaguanidine could be attributed neither to its dimer formation nor to the iceberg formation in aqueous solution. Since bile salts did not affect its physico-chemical properties such as apparent partition coefficient and diffusion constant, and did not form micellar complex with the sulfa drug, it became evident that the absorption enhancement was not caused by their intraluminal effect. On the absorptive surface, bile salts did not increase the affinity of the sulfa drug to the intestinal mucosal surface. The exsorption study demonstrated that such enhancement was caused by the direct action of bile salts to the structure of the absorptive surface. This effect was reversible and could not be explained only by the calcium depletion.