The microcirculation is the fundamental nutrient supply and waste removal system of all tissues. Recent improvements in spectrophotometric technique have made possible the noninvasive assessment of oxygen supply and utilization in the gastric mucosa. The authors have utilized such methods to assess gastric mucosal hemodynamics. The technique permitted further clarification of the roles of the gastric microcirculation, mucosal oxygenation, and acid secretion in the pathogenesis of stress ulcers in the stomach of rats. Furthermore, it provided important information on the function of gastric mucosal hemodynamics in the healing of gastric ulcers in man. The technique is described along with the authors' correlation studies between spectrophotometric data and other techniques for measuring gastric blood flow (hydrogen gas clearance and aminopyrine clearance methods and direct electromagnetic flowmeter techniques) and the prevention of ulcerogenesis.