Protein, and essential nutrient for the growth of children, is the chief nitrogenous substant in the serum of humans. For the purpose of establishment of the normal data for evaluation of nitrogen balance, we examined the serum levels of urea nitrogen, creatinine, uric acid and serum amylase activity as well as that of protein in 560 normal school children. The results are as follows; 1. The serum level of protein is 6.1+/-0.75mg%, albumin; 3.9+/-0.57gm%, globulin ; 2.2 +/- 0.77gm%, urea nitrogen ; 14+/-3.8mg%, creatinine; 0.5+/-0.20mg%, uric acid; 3.9+/-0.91mg%, and the serum amylase activity is 127+/-41.4% units/dl. And the reference rnage of the above data is 4.6~7.6gm%, 3.3~4.4mg%, 1.5~3.0gm%, 7~22 mg%, 0.3~0.7mg%, 2.1~5.7mg% and 45~210 units/dl, respectively. 2. There is evidence of age relationship in the serum level of urea nitrogen which tends to increase with age. 3. We observed sex differences in the serum levels of total protein and albumin examined and those of urea nitrogen and creatinine examined, female was predominant in the former and male in the latter.