A method coupling gas chromatography/negative ion chemical ionization mass spectrometry (GC/NCI/MS) with a rapid one-step purification on a Sephadex G-10 column was developed to measure free phenylacetic acid(PAA), the major metabolite of ,β-phenylethylamine (PEA), in small amounts of the rat brain. Recovery of PAA for the Sephadex G-10 column was 96.2±2.0%.Pentafluorobenzyl derivatives of PAA (PAA-PFB) and heptadeuterated PAA (PAA-d(7)-PFB) were determined by GC/NCI/MS. No interfering peaks were present on the mass fragmentogram. A calibration curve constructed from the observed peak area ratio of PAA-PFB to PAA-d(7)-PFB and known weight ratio was linear within the range of 1.25 to 40pg PAA, with a correlation coefficient of more than 0.999. In the rat striatum, PAA was determined with 'within batch' and 'between batches' coefficients of variation of 0.81% and 1.24%, respectively.While PAA was significantly decreased by 40% of controls following treatment with pargyline,the monoamine oxidase inhibitor, a remarkable increase in the metabolite level was found following treatment with PEA. Acute and chronic treatment with antidepressants had no significant effects on PAA levels in the rat striatum. The present method should be useful in future investigations of the functional role of PEA in the brain.