In the pathogenesis of peptic ulcer, development of tissue hypoxia holds a prominent place. Its timely detection and medicamentous correction necessitates development of criteria for its diagnosis. A study was made of activity of the key enzymes of glycolysis and purine metabolism lactatdehydrogenase and adenosinedesaminase in tissues, erythrocytes, and blood plasma in patients with peptic ulcer both in its uncomplicated course and in development of complications. Correlation has been established between activity of enzymes and severity of the illness. Changes in activity of enzymes will, we believe, be helpful in diagnosing hypoxic states and disturbances in the immunological vigor underlying the ulceration.