Intragastric pH was continuously monitored in 21 patients who underwent colorectal surgery. Monitoring was started before surgery, and was continued for two days after surgery. Intragastric pH tended to increase during surgery, compared with measurements obtained before and after surgery, but was not affected by the duration of anesthesia or of the surgical procedure, or surgical position. After surgery, patients were divided into two groups: the cimetidine group (10 patients) received intravenous cimetidine 200 mg 4 times a day, while the control group (11 patients) received no treatment. Postoperative intragastric pH was higher than 3.0 throughout the study in the cimetidine group, but was approximately 1.3 in the control group. Upper gastrointestinal bleeding occurred in 2 patients in the control group, with intragastric pH falling abruptly during the bleeding episode. To prevent post-operative upper gastrointestinal bleeding, in addition to the administration of H2-blockers or antacids, appropriate treatments in response to changes in intragastric pH are necessary. Continuous monitoring of intragastric pH in surgical patients is considered to be of clinical importance.