Adaptation to oxidative stress is an improved ability to resist the damaging effects of reactive oxygen species, resulting from pre-exposure to a lower dose. Changes in uric acid and glutathione levels during ice-bathing suggest that the intensive voluntary short-term cold exposure of winter swimming produces oxidative stress. We investigated whether the repeated oxidative stress in winter swimmers results in improved antioxidative adaptation. We obtained venous blood samples from winter swimmers and determined important components of the antioxidative defense system in the erythrocytes or blood plasma: reduced and oxidized glutathione (GSH and GSSG), and the activities of superoxide dismutase (SOD), glutathione peroxidase (GPx) and catalase (Cat). The control group consisted of healthy people who had never participated in winter swimming. The baseline concentration of GSH and the activities of erythrocytic SOD and Cat, were higher in winter swimmers. We interpret this as an adaptative response to repeated oxidative stress, and postulate it as a new basic molecular mechanism of increased tolerance to environmental stress.