An assay, based on pre-column derivatization and micro-high-performance liquid chromatography-tandem mass spectrometry, was developed for the determination of the GABA(B) agonist CGP 44532 in rat plasma. CGP 44532, a highly polar 3-amino-2(S)-hydroxypropylmethylphosphinic acid, presented difficulties in developing a chromatographic method for the analysis of the compound in rat plasma. Instead of analyzing the target compound directly, it was derivatized prior to separation to a 4-nitrobenzylcarbamate isopropyliden derivative. In order to reach the required quantitation limit, on-line solid-phase extraction was utilized for sample clean-up and reversed-phase micro-column high-performance liquid chromatography, for separation of the plasma samples. The separated compounds were detected by negative electrospray tandem mass spectrometry in selected reaction monitoring mode. The derivatives show good chromatographic and mass spectrometric properties and both the target compound and the internal standard, could be eluted as symmetrical peaks with good signal/noise ratio. The MS-MS detection was selective and sensitive due to the straight fragmentation pattern. After injection of 200-microl sample aliquots, the limit of quantification was 10 ng ml(-1). The analytical assay is usable in the range of 10-500 ng ml(-1).