It was shown that during short (3–4 day) vegetable and meat diets the adaptation of proteolytic properties of the gastric juice to the quality of food becomes, to a certain extent, inert. Immediately after the change from the meat diet to vegetable and vice versa, protcolytic properties of the gastric juice correspond to the quality of the food, while in the following few hours — to the quality of the previous diet. Adaptation to the new diet is completely developed in 2 to 4 days. However, a paradoxical phenomenon on the 3th to 50th day of prolonged diet is noted, i.e. in meat diet — the phytolytic properties of the gastric juice become prevalent, while in vegetable—zoolytic.