A high-performance liquid chromatography assay for fluoxetine and its major metabolite, norfluoxetine, was established. Serum concentrations of the two were measured in 13 depressed outpatients following a 6-week trial of the drug. No significant correlations were obvious between clinical improvement as measured by the Hamilton Rating Scale for Depression and the Clinical Global Impression scores and the serum concentrations of fluoxetine and its major metabolite, norfluoxetine, according to these initial pilot data.