It has been suggested that the features of saliva (e.g. fluidity, secretion and amino acid concentration) reflect physiological and psychological state of primates as well as subprimates, however, studies which revealed the relationship between the circadian rhythm and the concentrations of salivary amino acids have been limited. In order to better understand their physiological role, diurnal changes of salivary amino acids were investigated in three undergraduate students. Salivary amino acids were recovered after deproteinization with 5% trichloroacetic acid and determined by an amino acid analyzer. Most amino acids, except for methionine, cysteine and asparagine, were detected in the saliva. The intake of lunch or amino acid supplement transiently increased the salivary amino acids, and in the latter case, the amino acid levels returned to baseline within 10 minutes. Physical exercise also slightly elevated the salivary amino acid levels. During the university examination period, the secretion of saliva was slightly, but not significantly, increased, accompanied by the elevation of glycine, alanine, ornithine, histidine and threonine, and the decline of lysine, leucine, aspartic acid and hydroxyproline. Salivary amino acid levels may be useful to evaluate stressful conditions.