Anesthetized cats were cannulated and successively bled to arterial pressures of 100, 75 and 50 mmHg. The pressure was maintained at each level for 20 min. One group of animals was given 16 mu . kg-1 . h-1 of pentagastrin and another group 2 mg . kg-1 . h-1 of cimetidine throughout the experiment. Blood flow was determined by means of the microsphere distribution technique using spheres 14 +/- 1 micrometer in diameter. At each pressure level 15 min sample of gastric juice were collected. Twenty ml of saline was instilled into the stomach and withdrawn together with the gastric juice. Adenosine triphosphate (ATP) content and enzymatic activity (LD, ASAT) of the gastric mucosa and blood pH, pCO2 were determined at 140 and at 50 mmHg. In both pentagastrin and cimetidine treated cats gastric mucosal blood flow decreased and the vascular resistance increased markedly when the pressure was lowered by bleeding from 140 to 100 mmHg. On further bleeding to 50 mmHg only slight additional flow reduction occurred and the vascular resistance remained constant. Under shock blood flow was higher and the resistance lower in pentagastrin than in cimetidine treated animals. In pentagastrin treated animals a parallel decrease in acid secretion and mucosal blood flow occurred under shock. Cimetidine inhibited acid secretion both at normal blood pressure and under shock. The ATP content of the mucosa did not decrease, no morphological changes of the mucosa occurred and there was no leakage of cellular enzymes during the period of hypotension.