The present study was undertaken to determine the plasma antidiuretic hormone (adh) during ether anaesthesia and surgery by utilizing a bioassay technique. The measurements of osmolality in serum and urine, and of urine output, were performed simultaneously. The present study clearly demonstrated a remarkable participation of adh in decreased urine output and in elevation of urinary concentration in 11 patients. Plasma adh level was increased markedly from 2.9 ± 0.93 to 16.9 ± 3.5 μu/ml, which was a sixfold elevation from preinduction concentration after 30 minutes of ether anaesthesia alone. This was followed by gradual decline almost to preinduction levels in two hours after the start of anaesthesia but before the operation. Plasma adh levels increased again to 34.5 ± 11.7 μu/ml ten minutes after the start of operation. A decrease in urinary output and an increase in urinary osmolality were also observed. The increase in plasma adh level was less marked during deep ether anaesthesia. In operations on the extremities or in orthopedic procedures, elevation in the plasma adh level was less prominent than in laparotomy. Our study clearly indicates that the raised ADH secretion during anaesthesia and surgery contribute in part to the decreased urine formation.