Liver mitochondria obtained from rats exposed to either 3 per cent or 5 per cent halothane had significantly higher rates of oxygen uptake than did those of a corresponding control series. This increased respiration occurred both in the presence and in the absence of adenosine diphosphate. No significant change in rates of respiration was found with exposure to 1 per cent halothane. The respiratory control ratio and adenosine-diphosphate-to-oxygen values were not altered by prior halothane anesthesia at any dose level used. Electron microscopic study of the mitochondria of the control and experimental groups revealed no morphologie differences. Apparently, therefore, the inhalation of halothane, as administered under the conditions of this study, does not induce any long-lasting impairment of liver mitochondrial function.