The contamination of air of a 50 m3 operating room has been analyzed after three and half hours of halothane enaesthesia with a closed circle system and an opened system. The analysis has been made by gaz chromatography. Concentrations of Halothane found during the use of an opened system are 100 to 120 times the allowable norms in U.S.A. After using the closed circle system, the analysis of air does not reveale any trace of Halothane.