An original, highly sensitive and specific high-performance liquid chromatographic (HPLC) assay has been developed for the measurement of oxycodone in human plasma with a detection limit of 10 ng/ml using a 1.0-ml plasma sample. Plasma samples were initially acid-washed and then extracted twice at pH 10 with butyl chloride. Oxycodone and codeine (internal standard) were eluted with a mixture of methanol, acetonitrile, and 0.01 M pH 7 phosphate buffer to which 40 mg/l of cetyltrimethylammonium bromide (cetavlon) was added at ambient temperature and detected with electrochemical detection. The addition of cetavlon to the mobile phase markedly reduced the interaction between oxycodone and Si-OH groups on the stationary phase of the HPLC column, so that the organic content of the mobile phase could be reduced from 80 to 25%. Quantitation was achieved using the peak height ratio of oxycodone to codeine. The assay is currently being used for the study of oxycodone pharmacokinetics after single oral doses of 10 and 20 mg and single rectal doses of 30 mg.