Aim: To study the activity of enzymes of the gastrointestinal tract of different manufacturers at different temperatures and acidity in the conditions of an in vitro model fermenter. Methods: In a certified laboratory to determine the activity of enzymes using biochemical, physiological, analytical, metrological methods. Results: The activity of endo-1.4-glucanases and endo-1.4-xylanases was studied in 24 feed enzyme preparations in the conditions in vitro simulating the gastrointestinal tract of poultry. Lowering the temperature of the medium from 50°C to 38°C negatively affects the activity of all enzymes, which at pH 3.0 was almost 2 times higher than at pH 7.0. In general, the activity of enzymes containing xylanase was more stable compared to glucanase under conditions simulating both the stomach and intestine. Conclusion: The results obtained in the experiments in vitro allow us to conclude that the matrix values of enzyme activity indicated by manufacturers when labeling products do not reflect its effect in the organism of animals. Therefore, feed enzymes presented on the domestic market should be used only after scientifically based production studies. When developing recipes for compound feeds and premixes, which include enzyme preparations, it is also necessary to take into account the specific effect of feed enzymes on the type of raw material, changes in the digestive system in ontogenesis, and the functional features of various sections of the gastrointestinal tract of animals.