Exposure to extreme heat and cold is one of the environmental factors whose action is precisely based on the mechanisms involving free radicals. Fluctuations in ambient temperature are among the agents that toughen the human organism. The goal of the study was to evaluate the impact of extremely high (dry sauna, DS) and low (whole-body cryostimulation, WBC) environmental temperatures on the oxidant-antioxidant equilibrium in the blood of healthy male subjects. The subjects performed a single DS bath (n = 10; 26.2 ± 4.6 years) and a single WBC procedure (n = 15; 27.5 ± 3.1 years). In the subjects' blood taken immediately before and 20 min after the interventions, the activity of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx) and the concentration of thiobarbituric acid reactive substances in erythrocytes (TBARSer) and blood plasma (TBARSpl) were determined. Single WBC and DS procedures induced an increase in the activity of SOD and GPx, as well as SOD and CAT, respectively. The SOD activity was higher after WBC than after DS. Extremely high and low temperatures probably induce the formation of reactive oxygen species in the organisms of healthy men and, therefore, disturb the oxidant-antioxidant balance.