In order to evaluate comprehensively haloperidol pharmacokinetics under fixed-dose treatment conditions, psychiatric patients were studied after treatment with an acute dose, during maintenance therapy, and after withdrawal from haloperidol following steady-state conditions. After single doses, haloperidol appeared rapidly in serum, achieving peak concentration at a mean of 4.5 hours. The range of observed elimination half-life was broad, between 8.5 and 66.6 hours, with a mean of 19.5 hours. Under conditions of chronic dosing, serial measurements of steady-state serum concentration revealed intrapatient coefficients of variation between 2 and 72%. The mean for all patients was 26.4%. Body clearance decreased nonsignificantly, and elimination half-life increased significantly after chronic dosing compared with kinetic parameters determined after a single dose. The concentration of haloperidol in serum obtained at 8 hours after a single dose correlated most strongly (r= 0.73;p< 0.0001) with steady-state concentration resulting from chronic dosing. A value of 4 ng/ml or lower determined 8 hours after a single oral dose of 0.2 mg/kg identified patients who did not accumulate haloperidol during chronic dosing of 0.4 mg/kg per day above a presumed therapeutic range for haloperidol of 5 to 15 ng/ml. The implications of these data for the clinical use of haloperidol are discussed.