- Methods: With insitutional approval, 50 Sprague-Dawley rats participated in this study. All animals were anesthetized with halothane (3.5% inspired concentration) in a small chamber. A high fresh gas flow of 10 L/min was used to insure rapid equilibration of the ambient halothane concentration with vaporizer output. Rats were randomly assigned to one of 6 temperature groups (6-7 rats per group): 37, 38, 39, 40, 41 and 42 degrees C. Hyperthermia was produced by placing rats in a pre-warmed heating room. 30 min was required to achieve the target temperature (rectal temperature probe) which then was maintained for 1 h prior to determining MAC. MAC was determined using the tail-clamp method as previously described. Data were analyzed using one-way analysis of variance. Data are expressed as mean±SD.