A method was developed for the simultaneous determination of hypoxanthine, xanthine, uric acid and allantoin based on isocratic reversed-phase chromatography. This HPLC-method additionally allows the direct determination with UV-detection of inosine-5'-phosphate, uridine, thymine, orotic acid, allopurinol and oxipurinol, besides hypoxanthine, xanthine and uric acid in the same chromatographic run. Allantoin elutes in this system near the void volume and a fraction is collected covering the retention time range for this substance. After hydrolysis allantoin is converted to glyoxylate-2,4-dinitrophenylhydrazone, rechromatographed and detected at 360 nm. The coefficient of variation for this method does not exceed 5.0% for a serum concentration of 0.3 mumol/l hypoxanthine and is not greater than 5.3% for a xanthine concentration of 0.3 mumol/l serum. Recoveries were 90-110% for both hypoxanthine and xanthine. The determination of uric acid had an imprecision and inaccuracy not exceeding 1.45% in the concentration range of 103-568 mumol/l. Due to the more complex procedure required for the determination of allantoin, the coefficient of variation between days was 13.6% for a sample containing 0.8 mumol/l allantoin and the recoveries for this analyte were in the range of 86-93%. Reference ranges (mean +/- SD) determined on 171 serum samples from healthy adults were 12.7 +/- 6.6 mumol/l for hypoxanthine, 3.3 +/- 1.4 mumol/l for xanthine, and 15.7 +/- 7.9 mumol/l for allantoin. No significant age or sex dependence was observed. Uric acid concentrations were 320 +/- 55 mumol/l serum for men and 206 +/- 55 mumol/l for women.