During one year, free and total tryptophan, tyrosine and non-esterified fatty acids were determined in the blood of eight healthy volunteers. No clear seasonal rhythm appeared for any of these parameters. But over this period, the following significant correlations were found: total tryptophan-free tryptophan; total tryptophan-tyrosine; free tryptophan-tyrosine. For the individuals, the quotient tyrosine-tryptophan showed to be constant during this year.