This study was undertaken to examine the role of prostaglandins and histamine in the hyperemic response to gastric mucosal damage followed by H+ back-diffusion. Cat stomachs were exposed to 2 mol/liter NaCl for 10 min followed by luminal perfusion at pH 1. Hypertonic saline caused extensive (microscopic) damage to the surface epithelium, increased gastric mucosal blood flow, and increased release of histamine, PGE2, and 6-keto PGF1α (prostacyclin) into portal venous blood. The effect of indomethacin and histamine blockers (H1+H2) on the hyperemic response to acid back-diffusion was related to the depth of the mucosal injury and the region of the stomach. In the corpus, indomethacin enhanced mucosal injury. In areas with superficial damage, the hyperemia was inhibited by indomethacin and antihistamines and eliminated by the combination of both. In corpus areas with indomethacin-induced deep lesions, the blood flow was very high, and this hyperemia was partly inhibited by antihistamines. In the antrum the hyperemic response was reduced by antihistamines. Indomethacin increased the release of histamine into the portal venous blood (baseline recordings) and reduced basal gastric mucosal blood flow.