A procedure has been developed for measuring fluoxetine and its desmethyl metabolite, norfluoxetine, in serum by reversed-phase high-performance liquid chromatography (HPLC), with ultraviolet detection at 226 nm. Fluoxetine and norfluoxetine are isolated from serum by liquid-liquid extraction. They are then separated by HPLC and quantified, with reduced haloperidol as the internal standard. Fluoxetine, norfluoxetine, and the reduced haloperidol are separated from all interfering peaks in about 15 min. The standard curve is linear (r = 1.000) for both fluoxetine and norfluoxetine concentrations over the range of 25 to 800 micrograms/L. Between-run CVs for 60 and 200 micrograms/L controls (n = 8) were 6.8 and 4.1% for fluoxetine, and 8.8 and 6.2% for norfluoxetine, respectively. In a study of 24 patients with depression who were being treated with 20-60 mg of fluoxetine per day, fluoxetine and norfluoxetine concentrations in serum, measured during the last three weeks of treatment, were 47-469 micrograms/L and 52-446 micrograms/L, respectively.