Almost all of the norepinephrine as a neurotransmitter is released from and reincorporated into sympathetis nerve endings, except for some which escapes into the blood. Pursuant to the activity of the sympathetic nervous system, the adrenal medulla secretes both norepinephrine and epinephrine into the blood. Thus, the plasma catecholamine concentration is an index of sympathetic nervous system activity.Halothane is said to decrease the plasma catecholamine concentration, but there have been no reports concerning the mutual relationship between the halothane and catecholamine levels. In the present study, the blood halothane and plasma catecholamine concentrations were measured within 60 minutes of 1% and 2% halothane inhalation, and 30 seconds and 5 minutes after an electric stimulation following 60 minutes of inhalation. The blood halothane concentration increased up to 30 minutes after the beginning of halothane inhalation and leveled off thereafter in both the 1% and 2% inhalation groups. Thirty minutes after the beginning of inhalation, the blood halothane concentration was significantly higher in the 2% inhalation group (20 mg/dl blood) than in the 1% inhalation group (10mg/dl blood). The plasma epinephrine concentration was not significantly different between the 1% inhalation group and the control up to 60 minutes of inhalation. However, 30 seconds and 5 minutes after an electric stimulation following 60 minutes of inhalation, the epinephrine level increased significantly. In the 2% inhalation group, the epinephrine level decreased significantly below the control level 15 minutes after the inhalation was started, and from 30 minutes the level was even significantly lower than that of the 1% inhalation group. The blood norepinephrine concentration was not significantly different between the 10% inhalation group and the control up to 60 minutes of inhalation, but was significantly lower in the 2% inhalation group than the control after 30 minutes of inhalation. The norepinephrine level increased slightly in both groups after an electric stimulation, but not significantly above the respective level at 60 minutes. These results indicate that when the blood halothane concentration rises above 20 mg/dl, the catecholamine release, and the sympathetic nervous system response, are suppressed.