Background. The aim was to determine the effect of heat acclimation (HA) on oxidative stress (OxS) and inflammation in resting conditions and on the response pattern of these parameters to exhausting endurance exercise.Methods. Parameters of OxS and inflammation were measured in non-heat-acclimated status (NHAS) and after a 10-day HA program (i.e., in heat-acclimated status; HAS) both at baseline and after an endurance capacity (EC) test in the heat.Results. As a result of HA, EC increased from88.62±27.51to161.95±47.80minutes(P<0.001). HA increased OxS level: total peroxide concentration rose from219.38±105.18to272.57±133.39 μmol/L(P<0.05)and oxidative stress index (OSI) from14.97±8.24to20.46±11.13%(P<0.05). In NHAS, the EC test increased OxS level: total peroxide concentration rose from219.38±105.18to278.51±125.76 μmol/L(P<0.001)and OSI from14.97±8.24to19.31±9.37%(P<0.01). However, in HAS, the EC test reduced OSI from20.46±11.13to16.83±8.89%(P<0.05). The value of log high-sensitive C-reactive protein increased from-0.32±0.32to-0.12±0.34 mg/L(P<0.05)in NHAS and from-0.31±0.47to0.28±0.46 mg/L(P<0.001)in HAS.Conclusion. HA increases OxS level. However, beneficial adaptive effects of HA on acute exhaustive exercise-induced changes in OxS and inflammation parameters occur in a hot environment.