The mechanism of excretion of the antimicrobial ofloxacin in human scalp hair was investigated. When black and white hairs were taken from a patient with grizzled hair, who had been treated with ofloxacin, a much larger quantity of the drug was detected in the black hair. To study this difference and to elucidate the cause, ofloxacin (6,20, and 60 mg/kg/day, bid.) was administered ip for 5 weeks to albino and pigmented rats, whose backs had been depilated beforehand. In the last week of administration, the time-plasma concentration profile of ofloxacin was determined. One week after the last dosing, the newly grown hair on the depilated area was collected, and the drug concentration in hair was measured. The concentration in hair of the pigmented rats correlated significantly with the daily dose, area under the plasma concentration curve (AUC), and maximum plasma concentration ( Cmm) at steady state, whereas that in the albino rats correlated with the dose and C , , only, because AUC did not increase linearly with the dose in the albino rats. Moreover, the drug concentration in the hair of the pigmented rats was always much larger than that in the hair of the albino ones, although AUC and C , , did not differ greatly between both rat groups. The findings suggest that ofloxacin is excreted in the hair in relation to the dose administered, and that the mechanism of the excretion is closely linked with the presence of melanin.