Assay of 8-hydroxycarteolol (a metabolite of carteolol) was achieved using high-pressure liquid chromatography with electrochemical detection. Plasma or urine samples alkalinized by addition of a sodium carbonate solution were extracted with ethyl acetate or chloroform. The residues from evaporation of the organic extracts were redissolved in pH 2.1 phosphate buffer, and the solutions were chromatographed on a Partisil 10 SCX chromatographic column. The detection of 8-hydroxycarteolol was accomplished using an electrochemical detector. The procedure is rapid, specific, and highly sensitive. Reproducible results can be obtained, with relative standard deviations from analysis of replicate samples within +/- 8%. With 1-ml samples, the lower quantifiable concentrations of 8-hydroxycarteolol in plasma and urine are approximately 5 and 25 ng/ml, respectively.