OBJECTIVE To observe the effect of ranitidine on gastric acid, plasma endothelin, and calcitonin gene-related peptide (CGRP) in patients undergoing the brain operation, and to explore the possible pathogenesis of ranitidine on preventing from gastric mucosal injury under the stress. METHODS Thirty patients who underwent brain surgery were randomly divided into 2 groups: Fifteen patients in the control group did not use ranitidine and the other 15 in the treatment group received ranitidine 150 mg intravenously twice daily besides the routine therapy. We continuously monitored the gastric pH value from 4 hours pre-operatively to 72 hours post-operatively in the 30 patients. We also determined the plasma endothelin and CGRP levels of the patients at the 4th hour pre-operatively and at the 4th, 24th, and 72nd hours post-operatively. RESULTS In the control group there was no significant difference between the mean intra-gastric pH values pre-operatively and post-operatively (P> 0.05). In the treatment group the level of intra-gastric pH was much higher than that in the control group (P< 0.05). In the control group, the level of plasma endothelin significantly higher and the level of calcitonin gene-related peptide significantly lower than that pre-operatively (P< 0.01), but the level of plasma endothelin significantly was lower and the level of calcitonin gene-related peptide obviously higher in the post-operative treatment group than that pre-operatively (P< 0.01). CONCLUSION The brain operation obviously influences the endogenous plasma endothelin and CGRP levels, but its influence on the intra-gastric acid is not visible. Ranitidine can obviously decrease the level of intra-gastric acid, and improve the macrocirculation of gastric mucous membrane by decreasing ET and increasing the CGRP level.