A method has been developed for the simultaneous determination of antipyrine and its three major metabolites in plasma of patients with renal failure. Plasma samples (500 microl) were hydrolyzed with beta-glucuronidase/aryl sulphatase. The compounds, after addition of sodium chloride, were extracted with chloroform:ethanol (90:10, v/v) in acidic medium. Chromatographic conditions comprise a C18 column, a mobile phase with 30% methanol and 70% 0.25N sodium acetate buffer (pH 5.0), a total run time of 10 minutes, and ultraviolet absorbance detection at 254 nm. Confidence limits showed 0.5 to 40.0 microg/ml(-1) linearity (r2 = 0.999); 0.1 microg/ml(-1) HMA, 0.05 microg/ml(-1) antipyrine and NORA, and 0.5 microg/ml(-1) OHA sensitivity and absolute recovery >95%. Interprecision and intraprecision expressed as coefficient of variation were <10% for all compounds investigated. The assay shows to be suitable for pharmacokinetics and drug metabolism studies after administration of a single oral dose of 500 mg of antipyrine to a patient with hypertension and chronic renal failure (CL(CR) = 34.17 ml/min(-1); 1.73 m(-2)).