We measured plasma free amino acids in ten healthy adults (five males, five females, ages 26 to 54 years), at four times of day, under two dietary protocols. The design allowed us to measure the effects of some common physiological variables on the metrical distributions (mean, SD, skewness, kurtosis) of 80 values (in most cases), for each of 20 amino acids. Analysis of variance indicates that individuals in a “public” population possess “private” phenotypes for 18 plasma amino acids under typical external experiences; the exceptions were aspartate and taurine. The collective infradiem variation rarely exceeded 50% of the nadir value for any amino acid. We conclude that genotype is an important determinant of plasma amino acid phenotype in normal persons, and that there is striking homeostasis of plasma amino acid values in human adults adapted to normal daily activity.