1. We report a method for measuring the concentrations of haloperidol (HL) and its major active metabolite, reduced haloperidol (RHL), in human scalp hair. 2. Hair samples were obtained from 59 patients who had been taking HL at fixed daily doses for more than 4 months and whose compliance was good. A morning pre-dose plasma sample was also obtained from 48 of these patients. 3. The concentrations of HL and RHL in hair (ng mg-1 hair) correlated significantly both with the daily dose (micrograms kg-1 body weight) of HL (r = 0.682, P less than 0.001 for HL and r = 0.813, P less than 0.001 for RHL, n = 59) and with the trough concentration (ng ml-1) of the corresponding compound in plasma at steady state (r = 0.558, P less than 0.001 for HL and r = 0.563, P less than 0.001 for RHL, n = 48). The correlation coefficients were slightly higher using the sum of the concentrations of both substances in hair (r = 0.829 for the correlation with daily dose and r = 0.609 for that with trough concentration). 4. Hair from other patients, in whom the dosage of HL had been changed within a few months prior to sampling, was sectioned into 1 cm-long portions successively from the roots and the concentrations of both compounds in each portion were measured. Assuming a growth rate of 1-1.5 cm/month, a history of individual dosage could be deduced in all patients from the distribution of the drug and metabolite along the single hair length.(ABSTRACT TRUNCATED AT 250 WORDS)