Any factor which increases rebreathing (such as a decrease in inflow rate or an increase in ventilation) or increase uptake (such as an increase in solubility or in cardiac output) will lower the inspired anesthetic concentration. This in turn, will be reflected in an slower rate of anesthesia induction. In a circle absorption system, the inspired anesthetic concentration may be reduced from the inflowing concentration by rebreathing of anesthetic depleted gas. Semiclosed system, most widely used modern anesthetic system was tested to determine what effect the inflow rate had on the rate of rise of anesthetizing alveolar concentration of halothane and enflurane. The results were as follows: 1) High fresh gas inflow rate put the inspired anesthetic concentration close to that of inflow more rapidly. 2) In halothane anesthesia with 4 and 6 L/min of fresh gas flow, the anesthesia induction time was about 20 and 15 minutes respectively. But there was marked delay of the development of anesthesia by 2 L/min of fresh gas fiow. 3) In enflurane anesthesia, the anesthesia induction time was within about 10 minutes without respect to inflow rate. With these results, in general inhalation anesthesia with halothane or enflurane when a semiclosed technique and 100% oxygen are employed, a total gas flow at 4 to 6L per minute is recommended to establish a satisfactory induction and maintenance level of anesthesia.