OBJECTIVE To determine whether ranitidine a) increases the values of gastric intramucosal pH (pHi) in critically ill patients, as determined by tonometry; b) reduces the variability of these measurements. METHODS Prospective, double blind, randomized, placebo-controlled study. METHODS General Intensive Care Unit of a teaching hospital. METHODS Twenty-five critically ill, mechanically ventilated patients requiring arterial catheter and nasogastric tube. METHODS Tonometer placement; blind, random administration of intravenous ranitidine (50 mg) or placebo. RESULTS Tonometer saline PCO2 (PCO2i), arterial blood gases, gastric juice pH and pHi were determined immediately before, and 2, 4, 6 and 8 h after, ranitidine (12 patients) or placebo (13 patients). Ranitidine significantly increased gastric juice pH, but did not affect PCO2i or pHi; pHi was 7.34 +/- 0.14 before ranitidine, and 7.30 +/- 0.12, 7.31 +/- 0.11, 7.31 +/- 0.14 and 7.31 +/- 0.12-2, 4, 6 and 8 h, respectively, after ranitidine administration (p = 0.55). Ranitidine did not modify the coefficients of variation of PCO2i or pHi, either. No significant changes in gastric juice pH, PCO2i or pHi were observed in the placebo group. CONCLUSIONS In critically ill patients, ranitidine has no effect on pHi values, and does not increase the reproducibility of pHi measurements.