Ultrastructural changes in the gastric mucosa of 14 piglets subjected to transient hemorrhagic shock (3 hr duration; mean arterial pressure 40 mmHg) are described. After 30 min there was mucosal edema and extravasation of red blood cells. Microthrombi with degranulating thrombocytes and fibrin strands were seen in the capillaries. These changes were also seen in the antral mucosa where no subsequent ulceration usually occurs. At this stage the cells of the neck region of the gastric glands showed mitochondrial dilatation and their apical parts were seen to bulge into the lumen of the gastric glands. Other cell types were affected later and necrosis progressed from the surface into deeper parts of the mucosa. Mast cells and mucosal endocrine cells were relatively resistant, being mostly undamaged at the end of the shock period and later. The findings suggest that, during the shock, local formation of thrombi contributes to mucosal ischemia and ulcer formation, and that the cells at the neck region of the gastric glands form the locus of limited resistance to ulcerogenic effects.