The article presents a fragment of the comprehensive research of the intensity of protein metabolism and activity of the hydrolytic enzymes in the body of the duck broiler cross STAR 53 (heavy) in ontogenesis. In particular, we show that the nature of the changes examined indicators in the bird has age and organ-tissue features. In the studied tissues proteinase activity varies in the following order: glandular stomach˂duodenal chyme˂duodenal mucosa˂pancreas and the protein content and amino nitrogen increases in the order: glandular stomach˂pacreas<liver. At the same time, it found that the proteolytic activity of tissue of digestive organs affects the biosynthetic processes in the body and growing calves correlates with the daily growth of poultry body weight. In particular, the reduction of the digestive organs of the proteolytic activity (in the tissues of the glandular stomach, mucosa,duodenum and pancreas) during certain periods of growth and development of ducks weakens splitting nutrient feed substances causing insufficient intake of free amino acids and inhibition of protein synthesis in tissues.