Allantoin (All) is an oxidative end product of purines in mammals. The small amount of All present in human plasma or serum results from free radical action on urate and may provide a stable marker of in vivo free radical activity. Because free radicals have been implicated in the development and progression of atherosclerosis, this study focused on the metabolic compounds of the All pathway. We propose a new fast CE (CE/UV) method for the simultaneous determination of All, uric acid (UA), hypoxanthine (HX), and xanthine (X) in human plasma. These products were quantified in the plasma of patients with chronic renal failure before hemodialysis (n = 6), patients with chronic heart failure (n = 6) and controls (n = 6). The filtered plasma were diluted ten-fold before the direct injection in CE/UV (195 nm), which allows separating the four compounds in less than 13 min. The metabolites were detectable at concentrations of 0.3-0.6 micromol/L. The method was linear over the range 0.5-150 micromol/L for All, HX, and X and 10-1500 micromol/L for UA (r > 0.99). The analytical performance of this method is satisfactory with intra-assay CV < 3.4%, inter-assay CV < 5% (HX and X < 7%), and recovery (93-101%). The proposed CE-UV method appears to be a useful tool for studying physiological and pathological changes of HX, UA, and All levels in plasma samples, the latter being a possible indicator of free radical damage in vivo.