A rapid and sensitive high-performance liquid chromatographic method was developed for the analysis of coumarin, 7-hydroxycoumarin and its glucuronide conjugate in urine and plasma. This method was used to monitor the urinary excretion of these compounds following a single oral dose of coumarin (100 mg). This new method gives excellent chromatographic separation and includes an internal standard. The method was validated and shown to be both accurate and precise in the range 0.5-100 micrograms/ml.