This review has attempted to show on physiologic and pharmacologic grounds that assisted (or compensated) and controlled respiration are logical technics that should he used with certain exceptions in general anesthesia administered by a closed or semiclosed system. This is BO because all anesthetic agents, except nitrous oxide and ethylene, and the premedication sedatives depress the sensitivity of the respiratory center to carbon dioxide. This results in a respiratory acidosis with accumulation of carbon dioxide, which varies with the depth of anesthesia and with the agent, unless corrected by assisted or controlled respiration. In as much as excessive carbon dioxide is a narcotic agent that may collect without signs or symptoms and may add to the anesthetic depression during operation as well as cause certain of the post-anesthetic disturbances, it is advocated that the minute volume of respiration be kept up to or slightly above normal by assisted or controlled respiration. It is thought that slight respiratory alkalosis exposes the patient to no hazard.By guaranteeing an adequate respiratory exchange, assisted or controlled respiration reduces the tendency of hypoxia to develop because of respiratory depression; it may prevent the superimposition of anoxic anoxia upon the histotoxic anoxia caused by most anesthetic agents.The argument is developed that assisted and controlled respiration are safe technics, provided the anesthetist knows the physiologic effects of intermittent positive pressure in the airway, and provided that he avoids anesthetic overdosage by watching certain nonrespiratory signs that may be employed as a guide to the depth of anesthesia.It is advocated that assisted respiration be used whenever possible to augment spontaneous respiration except when the absence of respiratory movements provided by controlled respiration may facilitate certain operative procedures. We believe, in common with others, that the technic of controlled respiration is safer when control is achieved through hyperventilation to reduce the blood carbon dioxide tension below the normal, combined with mild or moderate depression of the respiratory center, than when control is gained by the use of deep anesthesia to cause profound depression of the respiratory center. (The current apneic technic using curare is as safe as the hyperventilation method and as easy or easier of application. To a limited extent it employs hyperventilation.)Assisted or controlled respiration, by keeping the pulmonary alveoli well expanded, many perhaps lower the incidence of atelectasis following operation.Clinical experiences with assisted and controlled respiration while using pentothal or ether, have confirmed the experimentally-determined effects of these two agents upon the respiratory center and upon the Hering-Breuer reflex. Pentothal tends to produce a marked depression of the respiratory center so that hypoxia, through stimulation of the chemoreceptors, becomes the dominant respiratory stimulus, while ether produces less marked depression of the respiratory center so that carbon dioxide continues as the chief stimulus for respiration. Pentothal exaggerates the Hering-Breuer reflex through depression of the respiratory center; ether inactivates the reflex by means of a synaptic block somewhere in the central nervous system, and possibly by depression of the peripheral sense organs that are stimulated by lung inflation.