An analytical method was developed for the determination in urine of 2 metabolites of diazinon: 6-methyl-2-(1-methylethyl)-4(1H)-pyrimidinone (G-27550) and 2-(1-hydroxy-1-methylethyl)-6-methyl-4(1H)-pyrimidinone (GS-31144). Two of the urine sample preparation procedures presented rely on gas chromatography/mass selective detection (GC/MSD) in the selected ion monitoring mode for determination of G-27550. For fast sample preparation and a limit of quantitation (LOQ) of 1.0 ppb, urine samples were purified by using ENV+ solid-phase extraction (SPE) columns. For analyte confirmation at an LOQ of 0.50 ppb, classical liquid/liquid partitioning was used before further purification in a silica SPE column. An SPE sample preparation procedure and liquid chromatography/electrospray ionization/mass spectrometry/mass spectrometry (LC/ESI/MS/MS) were used for both G-27550 and GS-31144. The limit of detection was 0.01 ng for G-27550 with GC/MSD, and 0.016 ng when LC/ESI/MS/MS was used for both G-27550 and GS-31144. The LOQ was 0.50 ppb for G-27550 when GC/MSD and the partitioning/SPE sample preparation procedure were used, and 1.0 ppb for the SPE only sample preparation procedure. The LOQ was 1.0 ppb for both analytes when LC/ESI/MS/MS was used.