We studied the incorporation of cocaine (COC), benzoylecgonine (BE) and ecgonine methyl ester (EME) into hair from blood. At first, the time courses of three drugs in rat plasma following i.p. administration of cocaine were investigated over 360 min. AUCs of COC, BE and EME in plasma were 14.2, 60.7 and 53.4 micrograms/ml/min, respectively. In contrast, the concentrations of the three compounds in hair were 16.4, 1.7 and 0.8 ng/mg. In spite of that, the AUC of COC in plasma was much lower than that of the other two compounds in plasma, the concentration of COC in hair was much higher than that of the other two compounds. The incorporation of COC into hair was much greater than that of BE and EME. If the incorporation of drugs from blood into hair is compared by [concentration in hair]/[AUC in plasma], that of COC, BE and EME is represented by 77:1.9:1. Our results suggest that the incorporation of drugs into hair from blood unexpectedly depends upon the physical properties of each drug.