Deliberately induced hypotension reduces bleeding in operative fields, therby facilitating the surgical manipulation of a highly vascularized lesion and enabling a better dissection to be made. Hypocapnia is a technique by which the regional cerebral blood flow is reduced, effecting a decrease in the intracranial volume. The monitoring of end-tidal CO2 tension(PECO2) is widely done since the amount of end-tidal CO2 tension reflects indirectly the value of the degree of arterial CO2 tension(PaCO2). During hypotension, increased physiologic dead space my produce the widened PaCO2-PECO2 gradient and this large gradient makes PECO2 an unreliable indication of PaCO2. There are many reports on hypotensive agents and techniques. Induced hypotension with halothane has been reported to be a relatively safe and useful method by Murtagh(1960) and Schettini, et al (1967). We reported 100 cases of halothane induce hypotensive anesthesia for intracranial aneurysm surgery in 1979. The present study reports concerning the hypotensive anesthesia for 259 cases of intracranial aneurysm surgery, which were performed at Severance Hospital of the Yonsei University College of Medicine from 1972 to 1985. We evaluated prospectively the PaCO2-PECO2 gradient with modern infrared capnographs during the induced hypotension of 25 cases, which was performed for intracranial aneurysm surgery at this hospital. The result of the retrospective and prostpective studies were as follows. A. Retrospective study 1) Halothane and enflurane were used as the primary anesthetics in 246 and 13 cases, respectively. Hydralasine, nitroprusside, and trimetaphan were supplementarily used for inducing hypotension in 29, 19 and 15 cases, respectively. 2) The mean arterial blood pressure of the lowest blood pressure in the induced hypotension group was 57.2±9.3 mmHg, and the mean arterial blood pressure of the highest blood pressures during the induction fo anesthesia was 111.3±20.8mmHg. 3) There was no significant difference in the perioperative hemoglobins, hematocrits, and serum electrolytes. 4) In the introperative period gas analysis revealed respiratory alkalosis(arterial CO2 tension and pH were 29.7±6.7mmHg and 7.485±0.078, respectively). In other values there was no significant change. 5) The mortality rate in 259 cases of intracranial aneurysm surgery was 6.2%. B. Prospective study There was no significant difference in the PaCO2-PECO2 gradients between in the hypotensive period (5.5±3.8 mmHg) and in the normotensiveperiod(4.3±3.4mmHg). In conclusion, the technique of using induced hypotension with the inhalation anesthetics, halothane or enflurane, is a safe and useful one to use in performing surgery for intracranial aneurysm, and end-tidal carbon dioxide tension can be used as an indirect measure of arterial carbon dioxide during the induced hypotension.