The present study examined the effects of a short-distance triathlon on the induction of DNA effects in peripheral leukocytes, urinary excretion of oxidized DNA bases, and frequency of micronuclei in lymphocytes of human volunteers. Induction of DNA effects was measured as increased DNA migration using the alkaline comet assay. Increased DNA migration was found in leukocytes of all individuals at different time points after exercise and revealed a biphasic pattern. Twenty-four hours postexercise, elevated DNA migration was found, whereas lower values were detected 48 h after exercise. Seventy-two hours postexercise, the maximum increase in DNA migration was found and baseline values were still elevated after 120 h. A modified protocol of the comet assay for the detection of oxidized DNA bases revealed no differences in leukocytes before and directly after the triathlon. Urinary excretion of 8-hydroxy-2'-deoxyguanosine remained unaltered during the 5 consecutive days sampled. No differences were found in the micronucleus-frequency in lymphocytes before or 48 and 96 h after exercise. Our data suggest that DNA effects detected with the comet assay in leukocytes of humans after exercise are secondary effects that do not originate from oxidized DNA bases and do not result in chromosome damage.