To study the pathogenic mechanisms of stress ulcers, we have studied the morphological changes (micro and macroscopic) occurring in the stomach and duodenum in Wistar rats submitted to a technique producing stress (space restriction and water immersion), as well as the changes produced by vagotomy and adrenalectomy done before stress. Severe vasoconstriction in the submucosa and other histologic signs (minimal stress signs) as well as the presence of intramucosal foci of necrosis and ulcerations (Maximal stress signs) allow us to attribute the origin of stress ulcers directly or indirectly, to ischemia. Previous truncal vagotomy prevents maximal lesions of stress but not the so-called minimal lesions, thus suggesting that the vagus nerve potentiates the lesions produced by ischemia. Adrenalectomy increases the presence and development of stress ulcers therefore suggesting that neither steroids nor catecholamines are involved in their pathogenic mechanism.