The impurities of diethyl ether are mainly acetic aldehyde and ether peroxide. Other impurities are sulfuric acid, sulphur dioxide, mercaptane and ethyl ester. It was believed that these impurities are produced during production and storage. When we use ether containing impurities, inhalation of excessive peroxide can cause salivation, profuse bronchial secretion, lung edema and pneumonia. Excessive aldehyde also irritates the mucous membrane and can cause lacrimation, photophobia, conjunctivitis, an oppressive feeling of the chest, severe cough, headache, unconsciousness, bronchitis and pneumonia. It is well known that the deterioration of ether is favoured by contact with air, heat and sunlight. There are two opposite opinions on reuse of ether. Baskerville(1910) claimed that it should not be used for anesthesia twenty-four hours after the container is opened. However Harry and David Gold(1934) showed that, in ordinary anesthetic ether cans which were opened many times and stoppered with cork, the contents remaining pure by very delicate chemical tests for the usual impurities, aldehyde and peroxide, during a period of months. In order to measure the impurities of ether, four brands of ether for anesthesia were randomly selected for analysis. Type of containers and date of production in each group are as follows: Group I: Kong Shin Pharmaceutical Co. LTD., so called Korean made ether, 140 ml in brown, bottle one month old. Group II: May & Baker LTD., made in England, 100g(140 ml)can. Group III: Mallinckrodt Chemical Works, made in U.S.A., (1/4)lb(113.4 gm) can. Group IV: Showa Co., so called Japan made ether, 160 ml in brown bottle, fourteen months old in three samples and four years and six months old in another three samples, They were analysed by chemical tests for peroxide, aldehyde and acetic acid just after the containers were opened(zero day) and one, two, four, six and eight days later. Ethyl alcohol was analysed by gas chromatography. The results were as follows; Ether peroxide: Threshold limit value for U.S.P. is 7. 1 microgram/10 ml ether (0.025 mg of hydrogen peroxide/25 gm ether), In group I and III, they were within this value from zero to eight days but in group II and IV, they exceed this value already on zero day. Aldehyde: Threshold limit value in U.S.P. is 0.007mg/20ml ether(formaldehyde 0.0005%). In group I, there was nothing on zero day but after twenty-four hours it exceed threshold limit value and increased day by day up to eight days. In groups II, III and IV, they exceeded this value already on zero day, and increased day by day up to eight days but were below maximum allowable concentration for U.S.P. Acidity: The incidence was slightly increased day by day up to eight days but all were within normal range. By gas chromatography, analysis showed 4% ethyl alcohol in group III and IV. On the other hand, to ascertain the interrelationship between the production of impurities and the existence of alcohol or H₂O₂ in ether, 3% alcohol(group A), 6% alcohol(group B), H₂O₂ (600 microgramg%)(group C), and H₂O₂ with 3% alcohol together(group D) were added to ether respectively. In four groups, immediately after one, two, four, six and eight days after the containers were opened, they were analysed for peroxide, aldehyde and acidity. Peroxide: This was increased in all groups just after adding, and increased furthur day by day up to eight days when the increase was marked. Especially in group D, it increased sharply. Aldehycte: It was increased in all groups just after acding, and there after increased or decreased irregularly day by day up to eight days by which time it had increased markedly. Acidity: It was increased in all groups up to one and/or two days, there after decreased day by day up to eight days. However none of the groups exceeded the threshold limit value of 0.4 ml of N/50 NaOH. It was concluded that: 1. Impurities in ether for anesthesia are influenced by type of container for storage, date of its production, and duration after container is opened. 2. Ether for anesthesia should not be used for this purpose, if the original container has been opened longer than twenty four hours. 3. The production of impurities in ether was influenced by the presence of alcohol in ether. 4. It would be better to analyse the ether for impurities by chemical tests prior to clinical use.