The concentration of ether gas or cyclopropane in the air of the operating room was analyzed. 1) The concentration of ether gas or cyclopropane in the air of the operating room was mostly below 0.1%. 2) The maximum concentration of ether gas or cyclopropane was 1/20 to 1/30 of the lower explosive limit and was much less than expected. 3) The distribution of the concentration of anesthetic gases in the operating room showed no remarkable difference up to the height of 1 meter from the floor. But it was markedly influenced by the distance from the anesthetic apparatus and diluted substaintially in the inverse proportion of it. 4) In case of leaking of gases from the pressure limiting valve or the mask of the anesthetic apparatus, the concentration of gases was enough to explode in the neibourhood of valve and mask. Therefore, the closed method must be used for the anesthesia with ether and cyclopropane.