The renin-angiotensin axis has recently been called the source of disproportionate splanchnic vasoconstriction during shock, and blocking this axis decreased gastric stress ulceration during swine cardiogenic shock. The present study tested whether the angiotensin converting enzyme inhibitor captopril would prevent stress ulceration when given after the onset of canine hemorrhagic shock, and whether any detrimental effects would result from enhancing splanchnic perfusion with captopril during hemorrhagic shock. We found that captopril treatment was associated with a decrease in gastric mucosal injury and with a marked decrease in systemic acidosis. Captopril enhanced blood flow to the small intestine, pancreas, liver, and spleen, but not flow to the stomach, during shock. Following the reinfusion of shed blood, the captopril-treated animals had decreased mean blood pressures and increased heart rates compared with untreated animals. We found captopril alleviated the stress ulceration produced by canine hemorrhagic shock, but concluded that the likely mechanism was alleviating systemic acidosis through enhanced perfusion of other viscera rather than a specific enhancement of gastric perfusion.