A sensitive and specific analytical method has been developed for the measurement of beta-phenylethylamine (PEA) in human plasma and rat brain extracts. The method involves solvent extraction of PEA with cyclohexane in the presence of amphetamine or phenylpropylamine (PPA) as internal standards. Automated precolumn derivatization with o-phthalaldehyde and 2-mercaptoethanol followed by reverse-phase HPLC separated PEA and PPA from endogenous interferences. Detection and quantification were carried out by amperometric detection at +0.75 V relative to a Ag/AgCl reference electrode or by coulometric detection with analytical cell potentials set at +0.29 and +0.50 V. The limit of detection for PEA was 10 pg and the limit of quantification in plasma was 60 pg/ml. The within-day and day-to-day coefficients of variation were 16.1% (n = 3) and 40.6% (n = 8), respectively, at a plasma concentration of 154 pg/ml and 15.2% (n = 5) and 28% (n = 10) at a brain extract concentration of 110 pg/ml. Basal endogenous plasma PEA concentrations of 335 +/- 255 pg/ml (n = 12, range 127-1002 pg/ml) were found for normal volunteers and single, daily doses of 24 mg but not 12 mg of the MAO-B inhibitor, mofegiline, were shown to increase plasma PEA significantly. Basal whole brain and striatal concentrations were 0.584 +/- 0.243 ng/g wet wt (n = 3) and 2.89 +/- 1.03 ng/g wet wt (n = 4), respectively. Statistically significant increases (5.7-fold) in rat whole brain PEA concentrations were seen 3 and 6 h following the administration of a single dose of 0.3 mg/kg mofegiline to rats.