The anesthesiologist uses a wide spectrum of drugs, including inhalational general anesthetics, barbiturates, benzodiazepines, narcotics analgesics and their antagonists, and neuromuscular blocking drugs. All of these drugs in sufficient dose impair the ventilatory response to chemical stimuli, and may cause inadequate gas exchange. The effect of depression of ventilatory control depends on the magnitude of depression and the coexistence of functional abnormalities in the respiratory system. The functional abnormalities are the result of preexistent pulmonary disease or other disease processes that impair respiratory function, the anticipated effects of major surgery (e.g., pulmonary resection), and the complications of anesthesia and surgery. From a functional viewpoint, the mechanisms of the effects of these disease processes on ventilatory control are: (1) interference with the neurophysiological control of automatic ventilation; (2) impairment of peripheral or central chemoreceptor function; (3) impairment of respiratory muscle function; (4) increase in the mechanical load to breathing as a result of increased resistance or decreased compliance of the respiratory system; and (5) increase in the ventilatory requirements as a result of ventilation/blood flow maldistribution, metabolic acidosis, or increased metabolic rate. As a result of current trends in the use of multiple drugs and controlled ventilation during anesthesia, the patient is at greatest risk during the early postoperative period in the recovery room. In addition to the functional abnormalities described above, the probability of impaired gas exchange and respiratory failure is increased as a result of impaired metabolism and elimination of drugs as a result of hepatic and renal insufficiency, and acute changes in acidbase status, which alter the ionization and distribution of drugs.