R--lipoic acid (RLA) is an endogenous organic acid, and works as a cofactor for mitochondrial enzymes and as a kind of antioxidant. Inclusion complexes of RLA with -, - or -cyclodextrins (CD) were prepared and orally administered as a suspension to rats. Among them, RLA/-CD showed the highest plasma exposure, and its area under the plasma concentration-time curve (AUC) of RLA was 2.2 times higher than that after oral administration of non-inclusion RLA. On the other hand, the AUC after oral administration of non-inclusion RLA and RLA/-CD to pylorus-ligated rats did not differ. However, the AUC after intraduodenal administration of RLA/-CD was 5.1 times higher than that of non-inclusion RLA, and was almost comparable to the AUC after intraduodenal administration of RLA-Na solution. Furthermore, the AUC after intraduodenal administration of RLA/-CD was not affected by biliary ligation or co-administration of an amylase inhibitor. These findings demonstrated that RLA was absorbed from the small intestine effectively when orally administered as a -CD inclusion complex, which could be easily dissolved in the lumen of the intestine. In conclusion, -CD inclusion complex is an appropriate formulation for supplying RLA as a drug or nutritional supplement with respect to absorption.