Acute gastric mucosal lesion (AGML) is well known as a pathology induced in animals by the injection of sodium cinchophen.The morphological changes in AGML have not been fully elucidated.The author attempted to examine the pathogenesis of AGML from the morphological standpoint of the microcirculation in the rat gastric mucosa through the vascular casting method and the ultrathin section method. Both types of specimens were examined with scanning type or transmission type electron microscopes.In the casting specimens, observation revealed narrowing and cessation of capillary blood flow in AGML areas indicating that alteration of microcirculation in the gastric mucosa was evoked.The morphological patterns of microcirculatory alterations in various AGML areas were conspicuously different. The author classified them into five types as follows ; 1) contracted capillary type, 2) apical mucosal type, 3) extensive mucosal type, 4) transmucosal type without affected collecting venule and 5) transmucosal type with affected collecting venule.These findings indicated that the microcirculatory changes occurred initially at the superficial region of the mucosa and that they gradually spread out in both vertical and horizontal directions in the mucous membrane. In some areas, a small amount of casting material leaked from the capillary lumens, though it was uncertain whether or not vascular ruptures actually occurred.In ultrathin sections, two different types of endothelial cells were identified in the capillary walls of the mucous membrane of rats, namely a continuous type and a fenestrated type. The distribution pattern of both types of cells was different, that is, the continuous type cells were more numerous in deeper area of the mucous membrane, while they gradually decreased in number toward the luminal surface.These results indicated that AGML was initially evoked and spread due to progress of the disturbance of microcirculation of the gastric mucosa in rats.