An acetyl-homologue-metabolite, present in concentrations of up to ten times those of the unchanged drug, in the plasma of patients receiving oral doses of acebutolol, is reported. Combined gas chromatography--mass spectrometry has been utilized to determine the structure of the acetyl-metabolite. This acetyl-metabolite has been measured as acebutolol by previously published non-specific methods for the determination of acebutolol in biological fluids. A specific and sensitive gas chromatographic method is described for the separate quantitation of acebutolol and its acetyl-metabolite in plasma and urine. Using our method, preliminary data on the disposition kinetics of acebutolol are presented. The difficulties in interpreting previously published pharmacokinetic data for acebutolol, based on a non-specific method of analysis, are emphasized.