1) Eighty four patients aged between 24 and 64 years were utilized for this study with 2% F-O inhalation for 25 minutes or with 2% F-O for 5, 10 and 15minutes followed by 1% F-O. The maximam B. P., pulse rate, and pulse pressure were analysed. Atropine was used in some cases as a preoperative medication. Another group of 15 patients, having been inserted a tracheal tube under consciousness, was checked on cardiac-output, -index, stroke volume, circulatory blood volume and total peripheral resistance both before and 15 minutes after 2% F-O anaesthesia. Bennett assistor was used in some cases. EKG observation was also done. 2) In case of 2% F-O anaesthesia. an immediate decrement of BP was found in value of about 40 mmHg. Atropine seemed to be little effective against the decrement. 3) A recovery of the BP decrement was observed when 2% F-O was replaced by 1% F-O. 4) Although bradycardia was observed during the anaesthesia of both 1% and 2% F-O, atropine could absolutely inhibit it. 5) The decreased pulse pressure at F-O anaesthesia is apt to be furthermore intensified by preoperative medication of atropine. This is one of the criteria against the regular routine preoperative medication of atropine and to suggest that the side-effects of F-O anaesthesia in circulation and respiration should be taken account under the base of controling the F-O concentration. 6) It seems to be apparent that the BP drop by F-O inhalation is mainly due to the central inhibition of vasomotor system with the secondary dilatation of peripheral vessels, in addition to the inhibiting mechanism of cardiac muscle which causes the decrement of cardiac output. 7) EKG obsevations reveal a vagotonic state with nodal rhythm at F-O anaesthesia in most cases. Ischemia of cardiac muscle manifesting flattening or invertion of T-wave can not be observed. The improvement of preoperative EKG would rather be found in a number of cases.