Few studies have used the vasopressin test to evaluate urine concentrating ability after sevoflurane anaesthesia. We performed a vasopressin test on the first day after operation to compare the effect of prolonged sevoflurane anaesthesia for orthopaedic procedures (n = 11) with that of isoflurane (n = 10). Mean doses of sevoflurane and isoflurane were 10.6 (SE 0.9) and 8.5 (1.5) MAC-h, respectively. Mean peak serum fluoride concentration in patients anaesthetized with sevoflurane was 41.9 (2.5 mumol litre-1 and exceeded 20 mumol litre-1 for approximately 20 h. Each group showed similar responses to vasopressin. There was no evidence of subclinical nephrotoxicity in patients given prolonged sevoflurane anaesthesia.