A gas chromatographic method using nitrogen phosphorous selective detection was developed for simultaneous determination of haloperidol and its metabolite, reduced haloperidol, in human plasma. Combelen was used as internal standard. The method involved extraction and trimethylsilylation followed by the injection of 2-4 ㎕ of benzene layer, which was used to dissolve the trimethylsilylated derivatives of haloperidol and reduced haloperidol, onto SE-54 column [5% phenyl methyl silica fused capillary column, 16m×0.22 mm (I.D.)×0.33 μm (coated thickness)]. The temperature of column oven was programmed from 200℃ to 300℃ at the increase rate of 10℃/min, and also the temperatures of injector and detector were set at 300℃. Helium was used as carrier gas and its flow rate was maintained at 30 ml/min. The detection was conducted with nitrogen phosphorous selective detector. The retention times for combelen, reduced haloperidol and haloperidol were found to be 9.14, 9.75 and 9.99 min, respectively. The detection limits for haloperidol and reduced haloperidol in human plasma were both 0.2 ng/㎖. The coefficients of variation of the intra-assay were generally low (below 9.8%). The mean absolute recoveries of added haloperidol and reduced haloperidol from plasma were 72% and 84%, respectively. No interferences from endogenous substances were found.