Deliberate hypotension in the surgery of intracranial aneurysm's used to diminish bleeding and render an aneurysmal sac slack to make clipping easy. It has been reported by Murtagh (1960) and Schettini et al. (1967) that deliberate hypotension induced with halothane anesthesia is a useful method. We are reporting clinical experience of halothane induced hypotension in 13 cases of intracranial aneurysm surgery. The results of clinical observation were as follows: 1. On the average, the systolic blood pressure lay between 60 and 80 mmHg. Hypctension by halothane was readily controllable by increasing and decreasing the inspired concentration of halothane. The use of vasopressors was not necessary to raise the blood pressure. Recovery from anesthesia was rapid. The mean of Mean Arterial Blood Pressure of 13 cases was 56.077 mmHg during the .hypotensive phase. The rate of fall of the mean arterial blood pressure was 1.862 mmHg per minute. 2. The common EKG finding was bradycardia associated with hypotension, but the pulse rate increased by raising the blood pressure. 3. The serum electrolytes, Na, K, Cl, and CO₂ combining power, checked in the pre and posto-perative period, showed no significant change. 4. Arterial blood gas study showed increased values for PaCO₂ and oxygen saturation during the hypotensive phase. The PaCO₂ was slightly lowered during hypotension. The pH was within normal limits. 5. The Hb and Hct, checked pre and postop ratively, showed lowered values in the postopertive period. During surgical clipping of the aneurysm, there were two cases of aneurysmal rupture, requiring blood transfusion; the amount of whole blood transfused was 500 ml to 1, 000 ml. 6. Urinary excretion, observed during hypoension, showed a decreased urine output at a systolic blood pressure of about 70 mmHg. 7. Postoperatively there was one death at the end of second week and the cause was thought to be cerebral infarction, not directly relnted to anesthesia.