This study was designed to investigate the differences in plasma levels of epinephrine, norepinephrine and dopamine during air-oxygen-enflurane anesthesia (AOE) and those during nitrous oxide-oxygen-enflurane anesthesia (GOE). These catecholamine levels were measured at 8 carefully defined points before and during hysterectomy or ovariectomy in two groups of each ten patients. Plasma levels of epinephrine decreased significantly immediately after intubation in both groups. In the AOE group, epinephrine levels increased significantly 30 and 60 minutes after the beginning of the operation. In the GOE group, however, epinephrine levels decreased significantly 15 and 30 minutes after the beginning of the operation. The difference between the two groups was statistically significant. Plasma levels of norepinephrine increased significantly after the beginning of the operation in the GOE group but no significant differences were found in the AOE group except the levels 60 minutes after the operation. The difference between the two groups was statistically significant 15 minutes after the operation. Plasma levels of dopamine remained unchanged in the two groups. Mean arterial pressure changed with intubation and surgical stress in the two groups. The arterial pressure was significantly higher 30 minutes after the operation in the GOE group compared with AOE group. Pulse rate increased significantly during anesthesia and surgery in the GOE group. It is concluded that AOE induces more increase of plasma epinephrine and less release of plasma norepinephrine compared with GOE.