The purpose of this study was to investigate the effect of post-challenge acute values of serum cortisol levels in elite and sedentary athletes before aerobic exercise. This study included 22 elite middle-distance runners from various clubs around Turkey and 16 voluntary non-athlete men. The Cooper test was applied to study subjects. Heart rate values and blood samples of the subjects were taken before and after the Cooper test. The blood samples were analyzed at Central Biochemistry Laboratory of Sahinbey Research and Application Hospital of Gaziantep University. The results showed statistically significant difference between sedentary and athlete groups in comparison of BMI (P<0.05). A statistically significant difference was also found in pre-exercise cortisol and potassium levels and post-exercise lactate and potassium levels between both groups (P<0.05). No statistically significant difference was detected between groups in comparison of other blood parameters (P>0.05). Again, no statistically significant difference was found between pre-test and post-test values of pre and post-exercise Cortisol and Potassium levels of both groups (P>0.05). Consequently, cortisol levels were same in both groups and were not affected by exercise. According to these results, it can be said that athletes' volume of air entering the lungs is higher than those who do not do sports, and the level of their anaerobic respiration is lower.