A Cell is a basic unit of the living body. It is separated from extracellular space by the cell membrane, which consists of lipid bilayers. When a drug is administered to the body, it contacts inevitably with the cell membranes. Therefore, its fate in the body must be strongly influenced by its liopphilicity which determines its degree of penetrability through the cell membranes. For this reason, it is necessary for us to know the boundary between lipophilicity and hydrophilicity of drugs from the pharmacokinetic point of view. In order to clarify this boundary, I tried to analyze the relationship between partition coefficients and several pharmacokinetic parameters of various drugs and found some regularities, represented as below; 1) Drugs (log P*>-2) are lipophilic from the viewpoint of volume of distribution. 2) Drugs (log P*<-1) are hydrophilic, whereas, drugs (log P*>0) are lipohilic from the viewpoint of the rate of excretion unchanged in urine.