12 patients of an intensive care unit on respirator with peritonitis respectively septic complications were treated with 300 mg/die Ranitidine or 2000 mg Cimetidine/die administered via perfusor infusion under the conditions of a randomized double blind study. Both groups under investigation were comparable in respect to age, sex and grade of risk of stress induced bleedings. With ranitidine under the dosage mentioned above, a prophylactic sufficient control, yet not a complete control of intragastric pH-value was accomplished. With Cimetidine as monotherapy, however, even under the high dosage of 2000 mg/die, no successful control of the intragastric pH could be achieved. With 3 patients even the combinations of 2000 mg Cimetidine/die and 2 X 10 mg Pirenzepine/die did not prove sufficient. Especially for intensive care patients with a difficult to regulate intragastric pH (patients with peritonitis, sepsis) Ranitidine according to our findings is to prefer to Cimetidine for the prophylaxis of bleeding of gastroduodenal lesions.