Enzymatic activity of horse radish peroxidase administered to the gastrointestinal tract of adult dogs and rats was assessed. It was discovered that in adult dogs, 30% of the protein reached the distal parts of the small intestine in a non-splitted form. As early as two hours after administering horse radish peroxidase over 15% of active enzyme was detectable in the gastrointestinal tract of adult rats. 0.5% of non-splitted enzyme was found in the rat large intestine.