Guinea pigs anesthesized with ether or cyclopropane were confined in a closed box and the concentration of anesthetic gas in the box was measured by interferometer with the lapse of time. 1) The gases in the closed box showed the highest concentration from 40 to 60 minutes later and the maximum concentration was about 0.16% in case of ether gas, but in case of cyclopropane the former was after 40 minutes and the latter was 0.21%. These concentrations were about one-twelfth of the lower explosive limits. 2) This condition corresponded to the case, that a human adult was confined in a room with a size of 7m3. Therefore, it is concluded that, the only anesthetic gases from expiration of patients cannot reach an inflammable concentration.