The author studied the changes of the plasma arginine vasopressin(AVP) levels in 10 brain surgery patient to evaluate the water balance during the postoperative period. The control group was 15 normally hydrated adult subjects. The plasma AVP levels were measured by the radioimmunoassays(RIA). The plasma and urine osmolality was also measured simultaneously using the conventional method. The followings were presented here-in as the results. 1) In control group, the average plasma AVP level was 2.00pg/mL with the range of 0.4 to 5.6pg/mL. There was relative constant relationship between the plasma AVP and the plasma osmolality. The osmotic threshold was 280.4mosm/Kg. 2) In preoperative brain surgery group, the plasma AVP level was similar to control group but the range was wider than control group. There was no constant relationship between the plasma AVP level and the plasma osmolality. 3) The plasma AVP levels were markedly increased in the postoperative brain surgery group. The average level was 10.99pg/mL ranging 0.4 to 47.50pg/mL. In spite of increased plasma AVP concentration, the plasma osmolality and urine/plasma osmolality(Uosm/Posm) ratios were relatively constant to normal control and preoperative brain surgery group. 4) The relationship between the plasma AVP level and the Uosm/Posm ratio was changed according to the plasma AVP level but above 3.0pg/mL it was nearly constant.