The concentrations of antidiuretic hormone in plasma and urine were determined in three groups of patients submitted to the same operative procedure. Seven (group I) underwent general anaesthesia with thiopentone, fentanyl and nitrous oxide and received an infusion of isotonic saline solution 5ml min-l. Seven patients (group II) anaesthetized similarly, received isotonic saline solution 15 ml min-1. In group III (five patients) anaesthesia was produced by extradural blockade. Surgery under general anaesthesia induced a significant increase in plasma and urine ADH concentrations which were not modified by the fluid load. Extradural anaesthesia suppressed almost completely the release of ADH during surgery. This effect of extradural anaesthesia could be related to the interruption of conduction along nociceptive neural pathways.