With the aid of ion exchange column chromatography we determined quantitatively the free amino acids in eccrine, thermal sweat. Sweat was collected (a) from the face of 27 healthy men and 26 healthy women (b) from the face, chest, armpits, shoulders, back, upper part of the abdomen, hypogastrium, forearms and thighs of the same individual and (c) from the face of the same individual at different times. Sweat was deproteinised by adding an equal volume of 5% sulphosalicylic acid. 1. The results showed not only a constant, qualitative amino acid pattern in sweat, but also a relative constancy among the individual amino acids. 2. The concentrations of the free amino acids in sweat showed significant, individual variations. Particularly high excretion rates were observed in the following amino acids: alanine, glycine, citrulline, histidine, ornithine, threonine and serine. 3. As compared to men, women had an increased excretion of all the examined amino acids in sweat from the face, except for cystine. Statistically significant higher excretion was seen within this sex-specific comparison for the following amino acids: alanine, citrulline, glycine, histidine, isoleucine, serine, taurine, threonine, tyrosine and valine. 4. Essential amino acids such as isoleucine, leucine, lysine, methionine, phenylalanine and valine and also cystine were always excreted only in small amounts. 5. Significant differences were also observed in the total amino acid excretion and in the individual amino acid excretions in sweat obtained from different parts of the body of the same person. 6. The amino acid concentrations determined in the sweat from the face of the same individual at different times showed a relative constancy as compared to the large differences of the amino acid concentrations determined in the sweat from the face of different individuals.