It has long been thought that intestinal absorption of most of the drugs proceeds by passive diffusion mechanism, in which lipid solubility of the drug molecule is a determinant factor. However, water-soluble natural compounds such as amino acids and sugars can move across cell membranes by the specialized carrier-mediated transport mechanisms. Although some drugs which are structurally analogous to natural compounds have been suggested to be absorbed by such transporters, no clear evidence for the involvement of carrier-mediated transport mechanisms has been obtained. In the present study, through the approach by means of the molecular cloning and functional expression of drug transporters as well as membrane physiological analysis for the drug transport across the intestinal epithelial cell membranes, participation of the carrier-mediated transport mechanisms for the drug absorption was clarified. They include peptide transporter, monocarboxylic acid transporter, anion antiporter, and P-glycoprotein. Most of them have a function for the uptake of drugs into epithelial cells, leading to the increased absorption of drugs, whereas P-glycoprotein excludes drugs into the lumen, thereby decreasing the apparent absorbability of drugs. A rat intestinal monocarboxylic acid-proton cotransporter, MCT1, and an anion antiporter, AE2, were suggested to contribute to the pH-dependent intestinal absorption of monocarboxylic acids such as benzoic acid, lactic acid, nicotinic acid, and valproic acid. An involvement of such pH-dependent transporters in the intestinal absorption of weak organic acids is important, because they may have an alternative mechanism against passive diffusion according to the pH-partition hypothesis. PepT1 cloned from rat intestinal epithelial cells as a peptide transporter was clarified to localize at the intestinal epithelia brush-border membrane and to function for the absorption of beta-lactam antibiotics by the proton-gradient energized mechanism. In contrast, P-glycoprotein functions for the secretion of drugs into the intestinal lumen, thereby decreasing intestinal absorption of an immunosuppressive, cyclosporin A and a 5-HT3 receptor antagonist, azasetron. These lines of studies on the clarification of carrier-mediated drug absorption mechanisms will provide new knowledge for the strategies to the enhancement of intestinal absorption of drugs.