An improved gas chromatographic-mass spectrometric chemical ionization assay for the quantitative determination of haloperidol in body fluids is presented. A fused silica, bonded phase capillary column, combined with negative ion chemical ionization (NICI), ammonia as a selective reagent gas and the monitoring of preselected characteristic ions (SIM), provide the combined sensitivity and selectivity necessary for reliable measurements in the low ng/ml range. The lower limit of detection was 0.1 ng/ml plasma and the calibration curve linear in the measured range of 0.1-5 ng/ml. In combination with the excellent imprecision and inaccuracy data and a recovery exceeding 90%, the method is very well suited for quantitative determinations of plasma samples generated during clinical studies, e.g. evaluating the pharmacokinetics and/or bioavailability/bioequivalence of haloperidol.