Three main factors have been well studied that are relevant to drug incorporation into hair: melanin affinity, lipophilicity, and basicity. These factors are important because they appear to profoundly influence drug transportation through bio-membranes and thus the incorporation and retention of drugs in hair. The proposed drug incorporation rates (ICR) into hair are measured as the comparison between the total amount of drugs in the blood and the total amount of drugs in the hair. This experimental model was designed to compare the ICR of many kinds of drugs administered to rats once a day initially, and collect blood samples. The newly growing hair samples were collected 4 weeks after the first administration. Melanin affinity, lipophilicity, and basicity were compared to the ICR of various drugs to see if identifiable patterns were present in the ICR that varied according to one or more of these physicochemical properties. Results showed that basic drugs were incorporated into hair much faster and at higher concentrations than neutral or acidic drugs, implying that basic drugs were likely to be detected in hair easier than neutral/acidic drugs if there are no other intervening factors. This suggests that it can be predicted if a certain drug is likely to be readily detected in hair by its structure. The drug incorporation into hair greatly depends on the melanin and especially eumelanin content of the hair, implying that drug concentrations are considerably affected by hair color. There are many chemical structural factors affecting drug incorporation tendencies into hair, which means that small changes in functional groups alter incorporation rates and influence the capability of detecting drugs easily or with difficulty. Drugs are very quickly incorporated into the hair bulb, and drugs in the hair root show peak concentrations within 6 to 33 hours after administration. 4 tables, 14 figures, 25 references