In a series of 45 consecutive duodenal ulcer patients (DU) the activities of 10 marker enzymes from the brush border, basolateral membrane, mitochondria, and lysosomes were determined by analysis of homogenized material taken with biopsy forceps through an endoscope from the antral and body part of the stomach. They were compared with the enzyme activities determined in controls with similar types of gastritis but without any evidence of peptic ulcer disease. All the DU patients had gastritis in the antral mucosa. In the body part, about 30% had gastritis. In the antral mucosa of DU patients the activities of the membrane and lysosomal enzymes were mostly increased when compared with the controls. In the gastric body mucosa of DU patients the activities of the lysosomal enzymes were mostly increased, whereas most of the membrane enzymes showed unchanged activities when compared with the corresponding controls. Monoamine oxidase activities were decreased or unaltered in both regions in these patients. The finding of enzymatic changes in the gastric mucosa of DU patients gives further support to an altered mucosal metabolism in these patients.