The purpose of the present study was to investigate whether high metabolic stress to skeletal muscle, induced by intensive exercise, would lead to an oxidation of urate to allantoin in the exercised muscle. Seven healthy male subjects performed short term (4.39 +/- 0.04 [+/-SE] min) exhaustive cycling exercise. Muscle samples were obtained from m. v. lateralis before and during the first few minutes after the exercise. Venous blood samples were obtained before and up to 45 min after the exercise. The concentration of urate in muscle decreased from a resting level of 0.26 +/- 0.023 to 0.084 +/- 0.016 mumol.g-1 w.w. (p < .05) during the exercise and then rapidly increased during recovery to reach the resting level within 3 min after exercise. The concentration of allantoin in the muscle increased from a resting value of 0.03 +/- 0.007 to 0.10 +/- 0.014 mumol.g-1 w.w. immediately after exercise (p < .05) and then decreased to 0.079 +/- 0.002 mumol.g-1 w.w. during the first 3 min after exercise (p < .05). Plasma urate levels increased slowly from 305 +/- 16 to 426 +/- 20 mumol.liter-1 at 45 min in recovery (p < .05). Plasma allantoin was 11.9 +/- 2.6 mumol.liter-1 at rest and by 5 min the level was more than twofold higher and remained elevated throughout recovery (p < .05). The present results indicate that urate is oxidized to allantoin in the muscle during exercise, probably due to generation of free radicals. Furthermore, the findings support the suggested importance of urate as a free radical scavenger in vivo.