The concentration of chlorpromazine (CPZ) in hair was measured to demonstrate its value as an index of individual dosage history and compliance. An animal study using pigmented rats was conducted to confirm the dose-dependent accumulation of CPZ in hair. The concentration of CPZ in hair, newly regrown on a denuded area of the back after the administration of CPZ for 3 weeks, was 4.6, 8.5 and 16.6 ng.mg-1 hair after daily doses of 1.25, 2.5 and 5 mg.kg-1.day-1, respectively, significantly correlated with the daily dose. The concentration of CPZ in black hairs collected from 23 Japanese patients, who had been taking CPZ in fixed daily doses (30-300 mg/day), ranged from 1.6 to 27.5 ng.mg-1, and was significantly correlated both with the daily dose and with the trough plasma concentration at steady state. Several strands of hair collected from each of 5 patients, whose doses of CPZ had been changed within several months before sampling, were cut into 1-cm pieces successively from the scalp end and the concentration of CPZ in each piece was measured. With the assumption of a hair growth rate of 1 cm per month, the individual history of CPZ doses in all patients could be deduced from the distribution of CPZ along the hair shaft. In 5 patients with grizzled hair the concentration of CPZ in white hairs was much lower (< 10%) than in black hairs, suggesting that the strong affinity of CPZ for hair melanin may explain the accumulation of CPZ in black hair.(ABSTRACT TRUNCATED AT 250 WORDS)