It has been said that the selective permeability across the blood--brain barrier depends on several physicochemical properties of drugs such as hydrophobicity, molecular weight, and hydrogen-bonding potential. In order to investigate quantitatively the relationship between the blood--brain barrier permeability and the hydrophobicity of drugs, we have measured the micelle/water partition properties (Pmic) by the MLC (micellar liquid chromatography) method using a gel filtration version and compared with the blood--brain barrier permeability. The thermodynamic aspects of partition were derived by separating the micelle/water partition coefficient (log Pmic) into the enthalpy term PH and the entropy term PS. It was found that the PH shows a good correlation to the permeability, although log Pmic fails to do so. The result means that PH, which is easily obtained from the in vitro experiment, can be used as an excellent standard in discussing the transport phenomena through the blood--brain barrier.