This preliminary investigation determined the plasma amino acid concentrations in 136 healthy subjects. The subjects were divided into four groups according to their ages: gr 1; 1-3 years, gr 2; 4-11 years, gr 3; 12-19 years and gr 4; 20-45 years. Comparing among the groups, the results showed that in younger children (age 1-3 years) the essential amino acids were slightly lower than the other groups. A wide range for most amino acids were observed. The ratio of essential (EAA): non-essential amino acid (NEAA) in very young children (1-3 years) was lower than children, adolescents and adults. These ratios were normal (> 0.5). The data have been compared with those from several studies performed in normal healthy subjects of different age groups, races and dietary habits. The results showed that most of the mean values of individual amino acid were higher than others except cystine. The EAA:NEAA ratio of each age group was comparable with other reports. The mean values of plasma amino acids and EAA:NEAA ratio in healthy subjects were determined for diagnosis, follow-up and prognosis of malnourished condition, abnormal amino acid metabolism and as a reference for other purposes.