The purpose of this study was to investigate the changes of physiological fatigue variables(blood lactic acid, ammonia, inorganic phosphate, pH, rectal temperature) following different exercise intensities and experimental conditions, and provide the scientific basis for effective evaluation of exercise performance enhancement by studying fatigue variables which correctly reflect the progress of fatigue. The subjects of this study were 15 high school and University freshmen athletes(mean age 16.9 years) who participated regularly in training programs for more than 20 hours per week. Each subject participated in five different experimental conditions which were : VO₂max, 85% of VO₂ max, 75% of VO₂ max, 85% of VO₂max after CHO supplyrnent and 85% of VO₂ max after BCAA injection. All experimental exercise condition were continued until subject stopped because of fatigue. Following order of experimental conditions, as listed above, mean exercise performance times were 15' 38" , 44' 51" , 96' 13" , 61' 14", and 54' 29". The shortest exercise performance was VO₂max experiment condition and the longest exercise performance was 75% of VO₂max condition. Mean blood lactic acid concentration at the end of each experimental conditions were as follows : 17.6 m㏖/L, 8.67 m㏖/L, 1.96 m㏖/L, 5.77 m㏖/L, and 7.59 m㏖/L. There were significant differences among these blood lactic acid concentrations(p＜0.05). Blood lactic acid concentrations were increased following exercise, but remained approximately at the resting level in 75% of VO₂max. Blood ammonia concentration at the end of each experimental conditions were increased. Mean blood ammonia concentration were 188.1 μ ㏖/L, 173.1 is μ㏖/L, 209.4 μ㏖/L, 177.8μ㏖/L, and 152.6 μ㏖/L, and there were significant differences among these blood ammonia concentrations(p＜0.05). Also, there were large individual blood ammonia level differences at rest and at the end of exercise. Mean blood pH's at the end of the experimental conditions were : pH 7.22, pH 7.30, pH 7.37, pH 7.36, and pH 7.29. The largest pH decrease was at VO₂ max condition and the smallest decrease was at the 75% of VO₂ max condition. Also, there were significant differences among these blood pH values at the end of all experimental conditions(p＜0.05), Mean blood inorganic phosphate concentration were increased : these values were 4.96 ㎎/㎗, 4.87 ㎎/㎗, 4.37 ㎎/㎗, 4.56 ㎎/㎗, and 4.58 ㎎/㎗. There were no significant differences among these blood inorganic phosphate concentration at the end of all experimental conditions(p＞0.05). Mean resting rectal temperature at each experimental conditions were 37.2℃ and no significant differences were found at all experimental conditions(p＞0.05). However, mean rectal temperature were increased 37.99℃, 38.98℃, 39.20℃ 38.95℃, and 39.1℃ Therefore, in all experimental condition, there was a large increase in rectal temperature after exercise. Also, except the VO₂max experimental condition, there were no significant rectal temperature differences(p＞0.05). Therefore, even though blood lactic acid, ammonia, and pH might be fatigue variables the absolute, values of these variables do not reflect the progress of fatigue. However, blood inorganic phosphate showed consistent accumulation at the all experimental fatigue situation and small individual differences. Thus, compared with other fatigue variables, absolute value of blood inorganic phosphate might possibly be a good indicator of fatigue progress. Also, absolute rate of elevated rectal temperature might possibly be an indicator of fatigue progress too. In conclusion, using blood inorganic phosphate accumulation and rectal temperature elevation, one can be evaluating the training status and predict potential ability of exercise perfornmance.