To investigate the effects of exercise intensity and short-term training on alterations in plasma uric acid, two series of experiments were performed using untrained male subjects. In series 1, 6 subjects (age 19 to 23 yr) cycled at 120% VO2max for 1 min followed by 4 min recovery until fatigue or until 24 repetitions had been completed. In series 2, 7 subjects (age 19 to 25 yr) cycled continuously at 65% VO2max for 2 h. In both experiments, short-term training was performed by repeating the exercise protocol for three consecutive days. In series 1, a progressive increase of 40% (P less than 0.05) was observed on day 1 in plasma uric acid concentration over the duration of the exercise. On day 2, pre-exercise values remained elevated over day 1 (mean +/- SD, 476 +/- 77 vs 352 +/- 30 mumol.l-1) and showed a further 23% increase (P less than 0.05) with exercise. Although resting uric acid concentrations on day 3 were elevated (P less than 0.05) over day 1, the exercise levels between day 1 and day 3 were not different (P greater than 0.05). In contrast, in series 2, prolonged sub-maximal exercise failed to stimulate increases in uric acid concentration either between days or within days. It is concluded that exercise intensity rather than total work output is a critical factor mediating increases in blood uric acid concentration. These results are consistent with the interpretation that uric acid formation may arise from purine nucleotide degradation and fast-twitch fiber utilization during conditions of high energy utilization.