BACKGROUND: It has been said that anesthesia and surgery tend to increse 'stress hormone' followed by reduction of GFR and urine flow. We have been noticed a polyuria during mastoidectomy with anesthesia. We hypothesized that a reduction of ADH secretion related to operative procedure might be a cause of a transient polyuria. METHODS: In 41 patients who were in ASA class I, mean arterial pressure (MAP), heart rates (HR), temperature, central venous pressure (CVP) were measured at before induction, just before drilling with irrigation, 30 minutes and 60 minutes after drilling with irrigation, and on arrival in recovery room by groups (room temperatured irrigating fluid and warm fluid were used in group 1 and group 2) during procedures. In 7 of 41, blood samples for antidiuretic hormone (ADH) and plasma osmolalities (Posm) were withdrawn at the same time points. In all patients, fluid were administered with 4 ml/kg/hour throughout the procedures. RESULTS: In group 1, mean urine volume were 5.0 and 6.4 ml/min during anesthesia and drilling with irrigation that was significantly more than in group 2 (3.6 and 4.2 ml/min). In 7 patients, ADH concentration was decreased during surgery compared to pre-induction level, while the Posm were in normal ranges. None of the MAP, HR and CVP showed significant changes. ADH concentrations were not significantly correlated to Posm. CONCLUSIONS: We suggest that a reduction of ADH secretion may have a major role in transient polyuria during mastoidectomy, which might be related to the mechanism that; 1) lowered temperature of hypothalamo-pituitary system by cold irrigating fluid may induce a transient ischemic changes of pituitary gland, 2) absorption of hypoosmolar irrigating fluid to central circulation may reduce central osmotic pressure.