A method for the determination of bupropion and its three major basic metabolites in plasma is described. Following an extraction from alkaline plasma into 1.5% v/v isoamyl alcohol in n-heptane, a portion of the acid-backwashed extract was injected onto a column packed with trimethylsilyl reverse-phase material and eluted with a phosphate buffer-acetonitrile (80:20) mobile phase containing an ion-pairing reagent and triethylamine. The compounds were detected with a dual-wavelength UV detector (214 and 254 nm) to optimize sensitivity and facilitate simultaneous detection. The method provides an absolute recovery of approximately 85% for bupropion and approximately 98% for the metabolites. Day-to-day reproducibility did not exceed 4.0% for all compounds. The detection limits were approximately 5 ng/mL for bupropion and 100 ng/mL for the major metabolites. The limit of 100 ng/mL for metabolite quantitation is imposed by the internal standard concentration selected for steady-state studies. In single-dose pharmacokinetic studies, 10% of the steady-state concentration of internal standard was used; this permitted a 10-ng/mL lower limit of detection. Steady-state plasma levels of bupropion and the metabolites from eight different patients are presented.