A simple, rapid, and sensitive method which allows us to simultaneously determine bromvalerylurea (BVU) and its three metabolites (3-methylbutyrylurea [MVU], alpha-(cystein-S-yl)isovalerylurea [CVU], and alpha-(N-acetylcystein-S-yl)isovalerylurea [AcCVU]) was investigated by frit-fast atom bombardment liquid chromatography-mass spectrometry (frit-FAB LC-MS). The LC-MS analysis was performed after the solid-phase extraction from tissue and urine samples with a Sep-Pak C18 cartridge. Tissue homogenates and urine were adjusted to pH 4.0 and applied to the cartridges. The retained BVU and its metabolites were eluted from the cartridge with 2 mL of acetonitrile/10 mM ammonium acetate buffer (pH 3.5, 50:50, v/v). The eluate was analyzed by LC-MS, which employs a semimicro type L-column ODS column. The proposed conditions are as follows: mobile phase A, 0.4% glycerol in acetonitrile/10 mM ammonium acetate buffer (pH 3.5) (5:95, v/v); mobile phase B, 0.4% glycerol in acetonitrile; elution mode, linear gradient, 100% A (5 min) to 100% B in 15 min; flow rate, 0.2 mL/min; split ratio, 1:40. Extraction recoveries of BVU and its metabolites were 91.90-97.79% from the spiked liver homogenate and 89.68-96.13% from the spiked urine. The detection limits ranged from 10 to 25 ng/g in selected ion monitoring mode.