The pulmonary acid aspiration syndrome has been well known in clinical practice of medicine. In 1946 Mendelson first reported aspiration pneumonitis in obstetric services, resulting from aspiration of gastric contents into the respiratory tract due to vomiting and regurgitation during general anesthesia. lt is suggested that the critical pH for the development of the acid aspiration syndrome in .humans is 2.50 Bannister and Sattilaro, 1962; Vandam, 1965). MacKrell and Schwartz (1969) studied the H⁺ secretory rate of isolated gastric mucosa and found that all anesthetic agents can produce a dose dependent decrease in H⁺ secretion to the point of total abolishment. Meanwhile, Christensen and Skovsted (1975) pointed out that pH changes of gastric contents by various anesthetic agents (hslothane, cyclopropane and fluroxene) were caused by the effects of general anesthetic agents on the autonomic nervous system, rather than by direct effects on the gastric mucosa, when the level of anesthesia depth was kept same. The present study was undertaken to in vestigate the effects of general anesthetic agents (halothane, diethyl ether and methoxyflurane) on the pH of gastric juice during clinical anesthesia. Sixty patients were studied, and they were divided into 6 groups according to the kinds of premedicant drugs and anesthetic agents administered. Gastric juice, aspirated through a Levine tube, was examined for pH and volume at regular intervals during the study. The results of this study were summerixed as follows; 1. During halothane anesthesia, the pH of gastric juice increased gradually as time passed, and in the group who had diazepam and atropine sulfate premedication, the pH value 30 minutes after indoction of anesthesia was 2.53±0.27, which was statistically significant (p<0.05) as compared to the control value (1.82±0.08). 2. In comparison between the groups who had diaxepam and atropine sulfate premedication and diaxepam premedicstion only during halothaae anesthesia, the pH value 60 minutes after anesthesia was 3.64±0.48 for the former group and 2.41±0.32 for the latter group. 3. During diethyl ether or Penthrane (methoxyflurane) anesthesia, the pH of gastric juice showed a little inconstant variation which was not significant statistically. 4. The volume of gastric juice showed a tendency to be somewhat less in amount in the group who had diazepam and atropine sulfate premedication as compared to the group who had diazepam premedication. From the data shown above, it may be concluded that during halothane anesthesia, especially with diazepam and atropine sulfate premedication, the pH of gastric juice can be maintained at a higher level than when under diethyl ether or methoxyflurane anesthesia.