A double blind study of 44 patients (24 placebo, 20 cimetidine) was undertaken to determine tha effect of cimetidine on the pH and volume of gastric fluid during anaesthesia. The pH was significantly higher on cimetidine-treated patients than placebo-treated patients at 0, 15, 30 and 45 minutes following intubation. At induction of anaesthesia, mean pH on cimetidine was 4.5 and on placebo mean H was 2.0. Approximately 70% of patients on cimetidine and 20% on placebo recorded a pH above 2.5 during anaesthesia. The reason for a pH recording of less than 2.5 in seven patients on cimetidine could not be ascertained. Mean volume of gastric aspirate was slightly lower in the cimetidine group at 0, 15, 30 and 45 minutes, but the differences were not significant.