We measured plasma concentrations of haloperidol (HAL) and its metabolite, reduced haloperidol (RHAL), by high performance liquid chromatography (HPLC) in 45 Japanese psychiatric patients receiving HAL. Plasma levels of HAL had a highly positive correlation with daily dose per body weight. Plasma RHAL/HAL ratios had also a dose-dependent relationship, but their distribution was nonnormal and a bimodal pattern with an antimode at 0.7 was apparent by probit analysis. There were 8 subjects (18%) with high RHAL/HAL ratios (mean = 1.26, SD = 0.41) and 37 subjects (82%) with low RHAL/HAL ratios (mean = 0.42, SD = 0.13). RHAL/HAL ratios showed little intraindividual variability (+/- 10.6%), while interindividual variability was large. This may suggest that pharmacogenetic factors are involved in the metabolism of HAL and RHAL.