The utility of blood meal and formaldehyde treated casein as protein supplements in the nutrition of sheep receiving approximately 50% of their total nitrogen from urea was studied. It was observed that protected protein supplements positively influenced wool growth and quality. On the basis of biochemical analysis of blood the mechanism of this effect is discussed. It seems that small protein supplements (about 6% of the total protein) are not only a source of amino acids in the duodenum but also influence the hormonal control in animals.