The purpose of this study was to examine the effects of serum ammonia and lactate accumulation following level and downhill running on creatine kinase activities and the ratio of cortisol and testosterone secretion. The running tests were performed on a treadmill for 30 min and all subjects(n=10) of present study participated in two different kinds of experimental conditions.: one was level running(LR) and the others was downhill running(DR) at a decline angle -14 % with their favorable running speed. There were significant difference in the total VO2 between LR and DR conditions(p<.05). In the mean blood lactate concentration at the end of the 30 min running exercise, there were significant differences between LR and DR conditions(p<.05). In the mean blood ammonia concentration at the end of 30 min running exercise, there were significant differences between LR and DR conditions(p<.05). The mean blood CK activity of DR was higher than that of LR group at the 12 h, 24 h, and 48 h post exercise, and there were significant difference between LR and DR conditions(p<.05). In the mean blood testosterone concentration, there were no significant difference between LR and DR conditions(p>.05). In the mean blood cortisol concentration and the testosterone/cortisol ratio, there were significant difference between LR and DR conditions at 12 h post exercise(p<.05). In the downhill running, CK activity in the blood as an indicator of muscle damage was a significantly higher at the 12-24 h post exercise, one of most effective blood fatigue factor on delayed muscle damage might be ammonia concentration and production during eccentric exercise. Thus, downhill running had a potential possibility of muscle damage and cortisol might played important role in the metabolic compensation for muscle damage