The effects of halothane anesthesia on liver function, as reflected by postanesthetic changes in serum glutamic pyruvic transaminase (SGPT) levels, were evaluated in 2 groups of 40 patients. All patients had 2 halothane anesthetics. In 1 group, both anesthetics were administered at 1 atmosphere pressure with 30% O2 in the inspired air. In the 2nd group, both anesthetics were administered at 2 to 3 atmospheres (absolute) of pressure with 97 to 98% O2 in the inspired air. While the SGPT levels remained within the normal range in all cases, there was a significant rise in SGPT levels in patients undergoing surgery under 1 atmospheric pressure. Patients undergoing surgery at 2 to 3 atmospheres pressure (absolute) had SGPT levels which remained at low normal values. The difference in response of SGPT in the 2 groups was statistically significant. We conclude that increased O2 tensions alter the normal response of SGPT to halothane anesthesia and that increased O2 tensions appear to protect the liver from possible adverse responses to halothane or its metabolites.