Molecular oxygen in adequate amounts is a basic prerequisite for normal tissue metabolism and function. The amount of molecular oxygen in tissue, or tissue oxygen tension, is, however, determined by a large number of different factors (Table). In a state as complex as clinical anesthesia, the majority if not all of these factors may simultaneously be altered and often may be altered in different directions. During clinical anesthesia not only may alveolar ventilation and oxygen tension in the inspired air be altered, but cardiac output, peripheral vasomotor tone, and tissue oxygen consumption may also simultaneously be changed. It becomes impossible to anticipate what effect, if any, a particular anesthetic agent or technique will have on tissue oxygen tension. It is likewise impossible to evaluate under clinical conditions whether tissue oxygenation is normal. Blood pressure, pulse, respiration, and color are inadequate for this purpose except under extreme conditions. A normal or