It is known how noticeable modifications may be induced in the high sulphur content proteins of wool, by enrichment of the diet of sheep with cysteine or sulphur amino acids. It is also well known how the oral ingestion of gelatin significantly increases hair diameter and the degree of hardness of finger and toe nails. To the end of verifying the possible stimulating selective action performed by cystine and gelatin in the keratinization processes of the hairy structures, it was controlled: amino acids content in hair of normal rat after oral ingestion of a controlled quantity of gelatin-cystine added in the diet. The same experiments have been carried out on rats in which the condition of sufference of the hair was induced by means of a biotin-free diet. From the obtained data it seems that gelatin-cystine, added in the diet of normal or suffering rats, helps to modify the cysteine content of the sulphur-rich hair proteins (+ 50% abt.). The diet supplement bring about a considerable increasing of CYS (+ 35%) with a decreasing of GLY (- 15%).