A high-performance liquid chromatographic procedure for the determination of cocaine and selected metabolites (benzoylecgonine, norcocaine and benzoylnorecgonine) from human and canine serum has been developed. The analytes are extracted from 0.5-ml serum samples using strong cation-exchange and octadecylsilane solid-phase extraction columns. Chromatographic separation was accomplished on coupled cyanopropyl and silica columns using acetonitrile-6.25 mM phosphate buffer, pH 2.9 (80:20 v/v) as eluent and a flow-rate of 1.1 ml/min. Low-wavelength UV detection (228 nm, cadmium lamp) allowed detection of 1 ng/ml for each analyte. The assay was linear from 0 to 200 ng/ml (r2 = 0.997-0.999, n = 15 for each analyte) and showed good precision (relative standard deviation = 0.66-13.8%, n = 6) and accuracy (98-115% relative recovery).