Background: Cytokines are small proteins secreted by white blood cells which play an important part of the immune system. These proteins are secreted due to inflammation which can be caused by infection, cancer and also exercise stress. In this regard the most controversial type of exercise is strenuous exercise. That is why this study focused on this type of exercise in order to bring more knowledge about its effect on immune system, cytokines in particular. Aim: The aim of this research was to study the effects of strenuous exercise on levels of circulating cytokines, and analyze the association of cytokines with baseline fitness and body composition variables. Methods: Thirty recreationally active and healthy male participants were cycling at 70% of their VO2max. Venous blood samples were taken before and after exercise. The serum levels of 37 cytokines were analyzed. Additional variables involved in this study are fitness and body composition as well as temperature measures. Statistical analysis tests used in this study were dependent t-test and Pearson correlation. Results: Interleukin-19 (IL-19) and Osteopontin (OPN) showed significantly lower levels post exercise, whereas APRIL/TNFSF13, sCD30/TNFRSF8, MMP-3, sTNFR1, and TSLP showed significantly higher levels after exercise compared with levels before exercise. Cytokines that showed an association with VO2max, peak power output, body fat percentage and body temperature at baseline were BAFF/TNFSF13B, MMP-2, Pentraxin-3, IFNa2, IL-29 and Osteocalcin. Conclusion: It is concluded that acute strenuous exercise change both pro-inflammatory and antiinflammatory cytokines levels, but it remains to be investigated for how long these changes are sustained, or modified by regular exercise training.