1. 1. Free amino acids and ninhydrin-positive substances of rainbow trout muscular tissue and skin were studied using ion-exchange automatic chromatography. 2. 2. In both tissue, 35 compounds were located and identified on the data. Identification was not performed for two other related substances. 3. 3. 1-methylhistidine and 3-methylhistidine were revealed together in muscular tissue. The last compound exists in large amounts and represents an important cellular effector. 4. 4. Quantitatively, glycine and alanine were found to be the two most important amino-acids measured in muscle. Their role in osmotic pressure regulation was considered. 5. 5. Skin total amino acid content is very low. High taurine and low proline concentrations are the main characteristics of this tissue.