In order to evaluate the ventilatory efficiency of the F-type breathing circuit, the authors compared the conventional anesthetic circuit using a 2-corrugated tube with the F-type breathing circuit through arterial blood gas analysis in 20 adult patients. There were no statistical differences between the two groups in arteral blood pH, PCO2 and PO2 and so the authors concluded that the F-type breathing circute could be substituted for the conventional anesthetic circuit using a 2-corrugated tube and even is superior in the case of head and neck surgery.