A sensitive and selective high-performance liquid chromatographic method for determination of intact cadralazine in human plasma or urine has been developed. The sample was buffered (plasma, pH 7.6; urine, pH 12.0) and mixed with internal standard before it was applied to an Extrelut-3 column. After adsorption, the column was eluted with chloroform, and the eluate was extracted with sodium acetate-hydrochloric acid buffer (pH 2.1). A 20-microliters aliquot of the aqueous phase was chromatographed on a 5-microns Spherisorb ODS reversed-phase column, with acetonitrile-phosphate buffer (pH 6.0, 25:75) as eluent. The quantitation was achieved by monitoring the ultraviolet absorbance at 254 nm. The detection limit was 0.03 nmol/ml in plasma and 5.00 nmol/ml in urine. The within-assay variation and the day-to-day reproducibility were less than or equal to 10% for plasma or urine standard samples. No interferences from possible metabolites or endogenous constituents could be noted. The utility of the method was demonstrated by analysing cadralazine in samples from one hypertensive subject on a therapeutic dose of the drug (7.5 mg orally).