Biopsies of the gastric mucosa were collected from two patients with Ménétrièr's disease before, during, and after treatment with cimetidine and studied for fibrinolytic activity. The fibrinolytic activity in the homogenates or in the potassium thiocyanate soluble fractions increased markedly during the treatment. Immunological characterization of the activity indicated that its major component was an endothelium-derived type of plasminogen activator, and that the increase in activity was caused solely by an increase in the concentration of this component. This suggests an increase in the vascularity of the mucosal layers during treatment thus reflecting a repair process.