Hydrophobic cyclodextrin (CyD) derivatives, such as 2, 6-di-O-ethyl-β-CyD (DE-β-CyD), 2, 3, 6-tri-O-ethyl-β-CyD (TE-β-CyD), carboxymethylethyl-β-CyDs (CME-β-CyDs) with different degrees of substitution, 2, 3, 6-tri-O-acyl-β-CyDs with different alkyl chains (C₁-C₁₂) were prepared and their chemical structures and physicochemical properties were elucidated. Furthermore, possible utilities of hydrophilic and hydrophobic CyD derivatives as modifiedrelease drug carriers were evaluated on the basis of in vitro/in vivo correlations. The results obtained in this study are as follows : (1) Hydrophilic CyDs such as 2-hydroxypropyl-β-CyD are useful as immediate-release type carriers for poorly water-soluble drugs such as nifedipine. (2) Hydrophobic CyDs such as ethylated and acylated β-CyDs can be used as prolongedrelease type carriers for water-soluble drugs such as diltiazem hydrochloride, buserelin acetate and molsidomine. (3) Enteric CME-β-CyD derivatives are useful as delayed-release type carriers, and also as stabilizers for prostaglandin E and carmofur which are labile under alkaline conditions. (4) Various release rates can be obtained by combining hydrophilic and hydrophobic CyD derivatives in appropriate mixing rations, e.g., double-layer tablets consisting of β-CyD complex and DE-β-CyD/CME-β-CyD complexes released drugs rapidly at an initial stage, followed by slow release. The combination of CyD derivatives and pharmaceutical additives was also useful to modify the release rate of various drug molecules.