The purpose of this study is to analyze the causes of elevation of plasma antidiuretic hormone (ADH) level during surgery and the relationship between urinary volume and plasma ADH level by measuring plasma ADH level of patients undergoing operation for esophageal cancer. The results obtained were as follows: The plasma ADH level was 4.0 pg/ml before surgery, 59 pg/ml after skin incision, 190 pg/ml after thoracotomy, and 276 pg/ml after vagotomy (right esophageal branch). Elevation of the plasma ADH level was partially suppressed by epidural analgesia combined with GOF anesthesia. The main factors that elevate plasma ADH level during surgery were pain at the skin incision, the manipulation of the pleura and vagotomy. The plasma ADH level was high (114 pg/ml) just after surgery and decreased to a normal level (4.3 pg/ml) in the morning of the 2nd postoperative day. Urinary volume was 74 ml/h before surgery, 95 ml/h just after surgery and 40 ml/h in the morning of the 1st postoperative day, and then, continued to gradual increase. There was no correlation between urinary volume and plasma ADH level during surgery until the 1st postoperative day. Elevation of plasma ADH level was not a primary factor of oliguiria during and just after surgery.