A study of plasma arginine vasopressin (AVP) concentrations in 16 patients undergoing uncomplicated cholecystectomy was carried out to determine the relative importance of the various stimuli to AVP secretion during and after an operation. Eight of the 16 patients were given a high epidural blockade from T4 and T12 and blood samples were taken before, at frequent intervals during and for two days after operation for determination of plasma AVP and osmolarity; systolic blood pressure was measured each time a blood sample was taken. In the 16 patients there was a 1.8-fold rise of plasma AVP after induction of anaesthesia (from 1.7 pmol/l median basal value to 13.1 pmol/l) and an 8.8-fold rise after skin incision, the highest concentration during operation being 102 pmol/l. There was no significant difference in AVP concentrations, urine volumes or plasma osmolality in the control and epidural groups. Systolic blood pressure fell in all patients during operation by between 14 and 54 per cent, the fall being significantly greater in the epidural group. Notwithstanding the evidence that certain nervous impulses do cause an increased secretion of AVP during operation, these results clearly show that in the absence of such stimuli there is still an increased secretion, almost certainly caused by a fall in the blood pressure.