Because numerous substances, endogenous compounds as well as xenobiotics, interfere with determination of uric acid and creatinine, we have devised a more nearly specific method by which we can simultaneously determine uric acid and creatinine in plasma by "high-performance" liquid chromatography. We used a mobile phase of ammonium acetate (30 mmol/L) and methanol (156 mmol/L) at pH 7.0 and a flow rate of 1 mL/min. We used a C18 reversed-phase column, and measured absorbance at 235 nm, the wavelength corresponding to the absorption maximum of uric acid and creatinine. Uric acid and creatinine could be determined in less than 5 min by directly injecting plasma diluted fivefold; use of a precolumn eliminated the need for deproteinization. We evaluated the precision, recovery, linearity, specificity, and limit of detection of the method and we checked for analytical interference by 37 currently used drugs. We compared results with those obtained by routine and reference methods.