It has long been recognised that exercise increases oxidative stress and induces oxidative cellular damage, as a result of imbalance between oxidant and antioxidant levels. A few studies have analyzed o,o`-dityrosine values over the time relative to the type of exercise, and there is no final conclusion about the change in this biomarker concentration, as well as whether and to what extent the concentrations correlate with training status. The aim of this study was to determine o,o`-dityrosine levels in urine samples relative to two different sport activities (swimming and rowing) before and after exercise. Twenty-three professional young athletes participated in the study. All participants were regular athletes with professional competition experience over five years. Urinary o,o`-dityrosine concentrations were analyzed by commercial enzyme-linked immunosorbent assay (ELISA), using a Dityrosine (DT) ELISA kit (Japan Institute for the Control of Aging, Nikken SEIL Co., Ltd.) at baseline on the day before training, 12 h after training, and 36 h after training. Data were analyzed by one-way analysis of variance (ANOVA) and linear regression analysis using Microsoft Excel Version 2010. Values of absorbance of o,o`-dityrosine, for all participants, showed a non-linear change with peak in the second urine fraction, 12 h after the training. We found a statistically significant difference in o,o`-dityrosine absorbance according to the analyzed groups in the examined time period, with linear-down type of reaction (P=0.019) and parabolic ‘open down’ type of reaction (P=0.044). Our findings point out a significant rise in o,o`-dityrosine concentration in professional athletes as a consequence of potent stimulus of oxidative stress.