BACKGROUND: An ominous sign is the oliguria frequently observed during an anesthesia. In order to elucidate the causes of oliguria during early stages of anesthesia, we observed renin, aldosterone, antidiuretic hormone (ADH), creatinine clearance (Ccr), fractional excretion of sodium (FeNa) and free water clearance (CH2O) before and 1 hour after anesthesia. METHODS: Twenty-four patients (ASA physical status I II) 20 to 60 years of age scheduled for elective surgery of low risk were anesthetized with N2O/O2/enflurane. According to the urine volume, we divided the patients into two groups (oliguria group and control group). The criteria for the oliguria group was urine volume less than 0.25 ml/kg during the first 30 minutes. Eleven patients were allocated to the oliguria group. Blood samples for sodium, creatinine, osmolality, renin, aldosterone, ADH and urine samples for sodium, creatinine and osmolality were collected before anesthesia and 1 hour after anesthesia. Ccr, FeNa and CH2O were calculated with equations. RESULTS: Before anesthesia, urine sodium (mEq/L), creatinine (mg/dl) and osmolality (mOsm/kgH2O) were significantly higher in the oliguria group than in the control group (153.4+/-15.8 vs 107.2+/-14.9, 75.5+/-10.9 vs 48.2+/-8.7, 543.7+/-27.5 vs 380.1+/-49.1, P< 0.05). Before anesthesia, ADH (pg/ml) was significantly higher in the oliguria group than in the control group (9.4+/- 3.6 vs 1.9+/-0.5, P< 0.05). One hour after anesthesia, urine sodium, creatinine and osmolality were significantly higher in the oliguria group than in the control group (170.1+/-14.6 vs 46.7+/-6.5, 71.1+/-6.9 vs 15.0+/-2.5, 557.5+/-27.5 vs 176.9+/-17.9, P< 0.05). CH2O (ml/hr) was significantly lower in the oliguria group than in the control group (-46.4+/-7.5 vs 112.5+/-23.9, P< 0.05). CONCLUSIONS: Our results suggest that the main cause of oliguria is dehydration during the early stages of anesthesia, so before anesthesia, appropriate hydration with free water is necessary to avoid oliguria.