The practice of closed-circuit anesthesia provides many advantages, including conservation of body heat and airway humidity, hemodynamic stability, minimization of operating room as well as environmental pollution, and a saving of up to 90% in consumption of anesthetics. However, the misunderstanding of the theory of the uptake of inhalation anesthetics has hampered the clinical practice of closed-circuit anesthesia. With proper recognition of the existence of the functional residual capacity and the alveolar membrane, a new concept of uptake of inhalation anesthetics has been developed: (1) uptake is dependent on the inspired concentration; (2) the functional residual capacity is a part of the anesthetic circuitry; and (3) uptake should be represented by 1-FA/FI. This physiologic and common-sense approach to understanding the processes of uptake of inhalation anesthetics greatly facilitates the practice of closed-circuit anesthesia. This new approach not only provides for a simple calculation of the minute to minute requirement for anesthetic vapors, depending on the desired inspired anesthetic concentration, but also enables one to practice closed-circuit anesthesia easily and safely with the currently existing anesthetic equipment.