To determine the mechanism involved, the incorporation rate (ICR) of drugs into hair was compared to melanin affinity, lipophilicity and membrane permeability. The following 20 drugs were tested ; amphetamine, methamphetamine, p-hydroxyamphetamine, p-hydroxymethamphetamine, cocaine, benzoylecgonine, ecgonine methyl ester, morphine, 6-acetylmorphine, phencyclidine, 1-(1-phenyl)-piperidinyl-cyclohexanol, methylenedioxyamphetamine, methylenedioxymethamphetamine, methoxyphenamine, O-desmethyl methoxyphenamine, benzphetamine, norbenzphetamine, deprenyl, lysergic acid diethylamide (LSD) and 11-nortetrahydrocannabinol-9-carboxylic acid (THCA). Their ICRs were represented as the ratios of the drug concentrations in rat hair to AUCs (the areas under the concentration vs. time curves) in rat plasma. Cocaine had the highest incorporation rate understood and there was a 3600 fold difference between the ICR of cocaine and that of THCA, the lowest drug. The melanin affinity of these drugs was determined by incubating a test solution with melanin at 36°C in the dark for 2h. After incubation and centrifugation, the drug concentration in the filtrate was determined by GC/MS or LC. The drug most affinitive to melanin was cocaine, followed by benzphetamine, phencyclidine, methylenedioxymethamphetamine and LSD. The correlation coefficient between ICR and melanin affinity of the 20 drugs was 0.947 (0.949 excluding THCA). Lipophilicity was calculated from the retention times of HPLC according to Kaliszan's method. Although the correlation coefficient between ICR and lipophilicity was very low (0.201), it rose to 0.770 by removing only THCA. The combination of melanin affinity and lipophilicity brought about a higher correlation (0.979) with the ICRs. Our data also suggested that the higher ICRs of basic drugs than neutral or acidic ones are strongly related to the membrane permeability of the drug based on the pH gradient between blood (pH 7.4) and hair matrix (acidic).