Enzymatic hydrolysis of casein was investigated in vitro. The hydrolysis involved two stages: 3h in the acidic medium in the presence of pepsin, and 2h in the alkaline medium in the presence of trypsin. All samples were dialyzed and nitrate or nitrite content was determined in the dialyzation fluid using Greiss colorimetric method after reduction of nitrates to nitrites. The interaction of nitrates and nitrites with the substrate of enzymatic casein digestion was evaluated. Decreased nitrite and nitrate concentrations in the dialyzation fluid were observed. The amount of nitrates and nitrites „combined" with the substrates of enzymatic casein digestion was 6% and 18% respectively.