Sodium glycocholate, sodium taurocholate, sodium deoxycholate, EDTA, sodium salicylate, sodium caprate, diethyl maleate, N-lauryl-beta-D-maltopyranoside, linoleic acid polyoxyethylated (60 mol) mixed micelles (all 20 mM) have been ranked in order of their effectiveness as enhancers of the absorption of drugs in the rat small intestine, by use of an in-situ loop model with phenol red as a model drug. Local toxicity in rats was examined by assessing protein and phospholipid release as biological markers. Of the absorption enhancers, sodium deoxycholate, EDTA and N-lauryl-beta-D-maltopyranoside were the most effective; sodium deoxycholate and EDTA, however, caused significant release of protein and phospholipids. N-lauryl-beta-D-maltopyranoside, on the other hand, did not damage the small intestinal membrane. Sodium taurocholate enhanced phenol red absorption from the small intestine and resulted in little or no protein and phospholipids release. Sodium salicylate, diethyl maleate and the mixed micelles had no absorption-promoting effects on phenol red. There was good correlation between the area under the plasma concentration-time curve for phenol red and the amounts of protein and phospholipid released in the presence of absorption enhancers. From these results it might be concluded that N-lauryl-beta-D-maltopyranoside and sodium taurocholate are effective absorption enhancers which have low toxicity levels at a concentration of 20 mM.