Metabolism of haloperidol in humans includes N-dealkylation to inactive metabolites and reduction to reduced haloperidol; reduced haloperidol is also oxidized back to haloperidol. A single 0.5 mg/kg (0.00133 mmol/kg) oral dose of haloperidol and reduced haloperidol was administered to seven Chinese schizophrenics in a randomized crossover manner separated by a 2-week interval. The clearance values for the different metabolic pathways of haloperidol and reduced haloperidol were determined. There were differences up to 10-fold in both oxidation and reduction capacities. There were also interindividual variabilities in the elimination of reduced haloperidol (0.37 +/- 0.20 L/hr/kg) and the N-dealkylation pathway (0.74 +/- 0.36 L/hr/kg). Four weeks after the single-dose study, the same patients also received haloperidol (10 mg) twice daily for 4 weeks. The reduced haloperidol to haloperidol concentration ratio (0.40 +/- 0.16) after the 4-week therapy is related to individual variabilities in the interconversion process and the N-dealkylation pathway of haloperidol.