||Compounds circulating in the blood may become incorporated in a growing hair shaft. Many drugs and their metabolites have been detected in hair including cocaine, amphetamines, opiates, nicotine, and barbiturates. However, the factors controlling the incorporation of these drugs into hair are not known. Acid-base properties, pKa, lipophilicity, protein binding and molecular weight could affect the concentrations of drugs found in hair. Likewise, the nature of the hair itself (i.e., pigmentation, hair structure) could affect hair incorporation. We hypothesized that differences exist between the relative incorporation of drugs and their metabolites into hair which will identify important factors in hair incorporation. The present studies were designed to test this hypothesis by examining drug and metabolite incorporation into hair after controlled systemic administration and by making comparisons between hair and plasma concentrations. The hair concentrations of codeine increased in a dose-proportional manner in Sprague-Dawley rats. Varying the route of administration (i.e., constant infusion vs intraperitoneal injection) did not influence hair concentrations. The metabolites of codeine, morphine and morphine glucuronide(s), were also detected in rat hair although the concentrations were lower than codeine concentrations. In addition to studies on codeine and metabolite incorporation into hair, different strains of rats were used to examine the role of pigmentation in hair incorporation. The concentration of codeine and morphine in hair was found to be greatest in Long-Evans rats (black hair) and least in Sprague-Dawley rats (white hair); Dark agouti rats (brown hair) were intermediate. These data show that pigmentation can influence drug incorporation. In vitro binding studies to hair from the same three strains of rats confirmed that pigmentation was responsible for the increased hair concentrations. Hair removed by plucking from Long-Evans rats was then examined from 1 hr to 14 days after codeine administration to reveal the rate of codeine and metabolite incorporation and the relative pigmentation effects on hair concentrations. In addition, phenobarbital, a weak acid, was incorporated to a much lesser extent than codeine, a weak base. Furthermore, in great contrast to codeine hair concentrations, phenobarbital concentrations were not found to be influenced by pigmentation. These studies have demonstrated that drugs enter hair in a dose-proportional fashion. Drug metabolites, both phase I and phase II, were found in hair. In addition, pigmented hair incorporated much greater concentrations of weakly basic drugs (i.e., codeine, morphine) but not weakly acidic drugs (i.e., phenobarbital). This research has also established the rat as a useful model for hair incorporation of drugs.