A rapid, selective and sensitive method was developed for the determination of antipyrine and its main metabolites in plasma, saliva and urine by an automated high-performance liquid chromatographic system. Using a MOS-Hypersil reversed-phase column with a phosphate buffer--acetonitrile mobile phase, baseline separation of antipyrine, its metabolites 3-hydroxymethylantipyrine, norantipyrine and 4-hydroxyantipyrine, and the internal standard, phenacetin, was achieved within 6 min. Factors regarding the accuracy and precision of the method and the stability of phase I metabolites during sample preparation are discussed, taking into account certain drawbacks of previously published methods. Based on the same chromatographic system a method was developed for the assay of 4,4'-dihydroxyantipyrine in urine. This compound is an important metabolite of antipyrine in the rat, representing 12.6 +/- 1.8% of the administered dose (n = 18).