At present new chemical entities, which have been synthesized and screened on the basis of genomics and/or proteomics, have been suggested to exhibit unfavorable pharmacokinetic behavior as drugs due to their large molecular weight and/or high lipohilicity. The in vivo pharmacokinetic properties of drugs are determined by their absorption (A), distribution (D), metabolism (M), elimination (E) and toxicity (Tox). However the molecular mechanisms for their pharmacokinetic properties are extremely complex and have yet to been sufficiently elucidated. We focus various kinds of carrier proteins (transporters), which are expressed in the intestine and the central nervous system in ADME-Tox research, and attempt to elucidate their roles in the intestinal absorption of drugs and the expression of their pharmacologic effects.