Changes in gastrointestinal mucosal blood flow were evaluated by index of oxygen saturation (ISO2) and index of hemoglobin concentration (IHB) measured with a reflectance spectrophotometer. This report examined the reproducibility of endoscopic measurements of ISO2 and IHB. Study 1: The everted stomachs of three anesthetized rats provided hands-on instruction (one teacher and three learners). Six sets of readings were obtained endoscopically (the mean calculated to give the measurement) at each level of gastric mucosal perfusion when gastric blood flow was varied by withdrawing blood from the carotid artery. Study 2: Fourteen duodenal ulcer patients with ulcer bleeding were transfused and stabilized. Two endoscopists (one teacher and one learner) took turns to obtain endoscopic ISO2 and IHB measurements at the margin of the ulcer and at the adjacent normal appearing mucosa. delta ISO2 was calculated as the ulcer margin value minus adjacent mucosa value. In study 1, the correlation coefficients between the ISO2 measurements of the experienced and those of the other three observers were 0.99, 0.97, and 0.97, respectively. In study 2, the correlation coefficients between the ISO2 measurements obtained at the ulcer margin and at the adjacent normal mucosa, and delta ISO2 obtained by the experienced observer and one of the three learners were 0.94, 0.97, and 0.94, respectively. Relative to the adjacent area, 79% of the duodenal ulcers studied had increased (+delta ISO2), and 21% had decreased blood flow (-delta ISO2) at the ulcer margins. IHB measurements were less reproducible, particularly at the ulcer margin. The measurements of ISO2 and delta ISO2 were reproducible in the everted rat stomach and in duodenal ulcer patients.(ABSTRACT TRUNCATED AT 250 WORDS)