IntroductIonIt is necessary to pay attention to the important internal factors, biological clock, and the effect of such factors on physiological as well physical functions especially in different times of day and night.Recent evidences (knowing the effects of time on physiological variables) show that the human body undergoes a lot of changes during the day and the night, and has a special ability in each hour [1].Reilly et al. [11,12] showed that reaction time, muscular strength, anaerobic power, and the amount of flexibility are significantly different in the evening than the morning. Such variation is probably due to the higher temperature of body and hormone secretion in the afternoon. This research was designed to study the changes in the response of testosterone and cortisol hormones in different times of a day to a continuous aerobic activity. Immunoglobulins are parts of immune system components, which are produced by the lymphocyte B in blood serum and tissue liquids in primates [7]. Stress is one of the stimulating factors of immune system, and physical activity can result in stress, which leads to changes in the system [2].On the other hand, cortisol secretion increase in stress producing EFFEctS oF trAInInG tIME on SEruM AbstrAct:The purpose of this study was to examine the effect of incremental continuous running as well as morning vs evening-time training on changes in serum immunoglobulins including IgA, IgG, IgM, testosterone, and cortisol hormones responses. For this reason, 28 male athletic students were purposefully selected and randomly divided into the two groups; morning-time training group (N=14, weight = 68.2 ± 9.8kg, age = 19.5 ± 1.6 years, training time = 7: 30 AM) and evening-time training group (N=14, weight = 63.8 ± 8.4 kg, age = 19.81 ± 1.24 years, training time =16:30 PM). The participants were trained according to an incremental continuous running program with a certain heart rate for two months (16 sessions). To determine the amount of serum immunoglobulins as well as cortisol and testosterone hormones, the participants' blood samples was taken twice, once 24 hours before the first training session and once 24 hours after training session. Then, a 12 -minute running-walking test was used to measure the maximal oxygen consumption. The results showed there were not any significant differences between the amounts of IgA, IgM, IgG serum of both groups in pre-test and post-test (i.e., respectively, p = 0.727, P = 0.068, P = 0.14). Also, there were not any significant differences between the amounts of testosterone and cortisol hormones secreted of both groups in pre-test and post-test. However, there were significant differences between the concentrations of cortisol hormone of the two groups in pre-test (P = 0.006) and post-test (P = 0.0001). More ever, the results also showed a significant difference between the pre-test of the morning-time training group and the post-test of the evening-time training group (P = 0.0001). The findings of this research indicate that cortisol hormone...