Preoperative cimetidine 300 mg or ranitidine in 50 and 100 mg doses were administered intramuscularly to 120 patients in a randomized double-blind study. The volume and pH of gastric aspirate samples obtained after tracheal intubation and before extubation were measured. The pH of gastric aspirate was higher following ranitidine 100 mg than ranitidine 50 mg or cimetidine 300 mg at both intubation and extubation (p = 0.006). In addition, fewer patients tended to be "at risk" of pulmonary aspiration syndrome (pH less than or equal to 2.5) after ranitidine 100 mg than ranitidine 50 mg or cimetidine 300 mg. Preoperative intramuscular ranitidine 100 mg was found to be suitable for use in protection against gastric aspiration syndrome.