The effect of premedication and three general anaesthetics on gastric content pH was investigated. Neither premedication with pentobarbital-atropine nor morphine-scopolamine given 1-2 h prior to anaesthesia appeared to affect the acidity of the gastric contents. Halothane invaribly increased the pH of the gastric contents; none of the seven patients studied had a gastric pH of less than 2.5 (mean 5.1) after 1 h of anesthesia. Cyclopropane uniformly maintained the acidity of the gastric contents; only one out of seven patients had a gastric content pH above 2.5 (mean 1.7) after 1 h of anaesthesia. This effect of cyclopropane in maintaining the pH of gastric contents was unaffected by the use of premedication and induction with thiopental. Fluorexene affected the pH of the gastric contents much less uniformly. Although the pH for the group as a whole gradually increased (after one hour from 1.7 plus or minus 0.2 (s.e. mean) to 3.1 plus or minus 0.7), some of the seven patients studied reacted to fluroxene with a constant low gastric content pH. The findings are discussed, and it is concluded that the risk of pulmonary complications in case of vomiting and aspiration upon emergence from anaesthesia is greater if the anaesthetic agent is cyclopropane or fluoroxene, than if it is halothane.