Ketamine produces an increase in the heart rate and blood pressure, but the precise mechanism is controversial. In order to obtain inslight into the mechanism by observing the influence of nitroglycerin (NTG) on the ketamine effect, ketamine was administered intravenously following sublingual administration of nitroglycerin in conscious patients. The results were as follows: 1) In the ketamine(2mg/kg) group(n=15), the heart rate and blood pressure increased significantly by as much as 25%. 2) In the nitroglycerin(1.2mg) group(n=12), the heart rate increased, but blood pressure decreased from 5 minutes after administration. 3) In the group which received ketamine(2mg/kg) 5min. after NTG(1.2mg)(N=15), the heart rate did not change, but the blood pressure increased significantly without attenuation by nitroglycerin. From the above results, it is suggested that in the ketamine effect control mechanism may also be participated in addition ot the peripheral mechanism involving norepinephrine release.