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The minimum alveolar concentration of anesthetic (MAC) necessary to prevent movement in response to a painful stimulus was relatively constant in dogs anesthetized with halothane. MAC varied over a two-fold range with the intensity of the stimulus, but appeared to reach an upper limit beyond which a further increase in intensity did not increase MAC. For the same stimulus MAC was constant from dog to dog. MAC was unaffected by duration of anesthesia, unaltered by hypocarbia or hypercarbia, by phenylephrine-in-duced hypertension or by mild hypoxia (Pao2 30 to 60 mm. of mercury). Hemorrhagic hypotension or marked acute metabolic acidosis reduced MAC by 10 to 20 per cent. Severe hypoxia (Pao2 less than 30 mm. of mercury) reduced MAC by 25 to 50 per cent.MAC appears to be a useful standard by which all inhalation anesthetics may be compared.