A simple, rapid, sensitive, and specific liquid chromatography-mass spectrometry (LC-MS) method was developed and validated for the quantification of amitraz and its metabolite in human serum. Both the compounds were extracted using monolithic silica spin columns with acetonitrile. The chromatographic separation was performed on a reverse-phase C(18) column with a mobile phase of 10 mM ammonium formate-acetonitrile. The protonated analyte was quantitated in positive ionization by mass spectrometry. The method was validated over the concentration range of 25-1000 ng/ml for amitraz and its metabolite in human serum. For both compounds, the limit of detection was 5 ng/ml. The method was applied to serum samples taken from an attempted suicide patient, and only small volumes of serum were required for the simultaneous determination of these compounds.