In vitro studies suggest that volatile anaesthetic agents may directly inhibit insulin secretion. It is unclear if supplementation of anaesthesia with isoflurane impairs insulin secretion. We performed a 5-g i.v. glucose tolerance test in 21 patients before and during anaesthesia which was maintained with either 1 or 2 MAC of isoflurane in nitrous oxide, or no volatile agent. The study was carried out before surgery to avoid the influence of hormonal responses to trauma. A significant glycaemic response occurred during both i.v. glucose tolerance tests in all three groups of patients. Serum insulin concentrations were measured and the acute increase in insulin concentration at 3 min and area under the curve for 15 min were calculated. Both variables decreased significantly in all three groups during the tests performed under anaesthesia compared with tests carried out before anaesthesia.