Thirty critically ill patients with mixed diagnoses underwent continuous intragastric pH monitoring for 72 hr while confined to a shock/trauma intensive care unit. The first 24 hr were monitored under no specific acid-suppressing therapy (placebo control). During the second and third consecutive 24-hr periods, patients received continuous infusion of intravenous ranitidine in the dose of 6.25 mg/hr and 12.5 mg/hr, respectively. Results of the placebo-control 24-hr study revealed that one third (N = 10) of the patients were gastric acid hyposecretors (24-hr median intragastric pH values above pH 4.0). In the normosecreting group (N = 20), both ranitidine schedules significantly elevated 24-hr median pH values, when compared to placebo (placebo 24-hr median intragastric pH 1.75; ranitidine 6.25 mg/hr 24-hr median intragastric pH 4.625, P < 0.0001; ranitidine 12.5 mg/hr 24-hr median intragastric pH 6.29, P = 0.0099). Five patients (18%) failed to adequately respond to the ranitidine 12.5 mg/hr dose (24-hr median intragastric pH < 4.0). These findings suggest that a significant percentage of intensive care unit patients are not in need of acid-suppressing therapy as prophylaxis against stress-induced ulceration. Conversely, other patients may require more intensive acid-suppressing regimens because of failure to respond to high dose H2-antagonist therapy.