An improved method for the simultaneous determination of cocaine, norcocaine, benzoylecgonine and benzoylnorecgonine using reversed-phase high-performance liquid chromatography with ultraviolet detection is described. Following solid-phase extraction, chromatography was performed using a column containing an octadecylsilica-coated packing, eluted with 6% acetonitrile in phosphate buffer, pH 2.1, and detected at 233 nm. Using 80-microliters samples, the detection limit is 18 ng/ml for benzoylecgonine and benzoylenorecgonine and 35 ng/ml for cocaine and norcocaine. The coefficients of variation range from 3.5% (benzoylecgonine) to 7.0% (norcocaine). The procedure has been applied to samples of guinea pig plasma, urine and amniotic fluid and human urine.