A comparative analysis was made of the aggressive properties of gastric juice secreted by normal people and those suffering from peptic ulcer in response to histamine and pentagastrin. Ulcerogenic activity of gastric juice obtained in response to histamine was shown to increase in peptic ulcer. The "pentagastrin" juice did not exhibit such an effect. Experimental study of the occurrence of gastric juice proteases in dogs revealed an increased occurrence in most proteases and their elevated number of the "histamine" juice in ulcer development and a decreased occurrence of proteases in the "pentagastrin" juice. Low aggressiveness of gastric juice from peptic ulcer patients during pentagastrin-induced stimulation of secretion appears to be related to the listed changes in the spectrum and occurrence of proteases.