Using gas chromatography, we measured the concentrations of amino acids in a group of 80 selected subjects: we present the plasma values which we obtained, as well as those of other authors. Various comparisons were made, according to the age and/or sex of the subjects. We found that the plasma levels of valine, leucine, isoleucine and proline in patients of any age, the levels of hydroxyproline and phenylalanine in subjects over the age of 45 and the levels of ornithine in subjects under the age of 45, are higher in men, while the level of threonine is higher in women under the age of 45 than in men of the same age. These sex differences are significant. Valine, leucine, isoleucine, hydroxyproline, phenylalanine and ornithine are significantly higher in the plasma of elderly men, while plasma ornithine is higher and plasma threonine is lower in elderly women than in young women. We also determined the normal values of urinary amino acids and were unable to demonstrate any differences in terms of age or sex. We defined four groups of amino acids, according to their clearance values. However, these groups do not correspond to those defined by the existence of renal tubular transport systems common to certain amino acids.