We have compared two groups of patients given low- or high-dose fentanyl anaesthesia. Arterial blood samples were collected for measurement of glucose, free fatty acids (FFA), glycerol, beta-hydroxy-butyrate, insulin, c-peptide, glucagon, human growth hormone (HGH), cortisol and adrenaline concentrations. After induction of anaesthesia, blood concentrations of most of these substances decreased. After the start of surgery the concentrations of cortisol, glucose, HGH, FFA and beta-hydroxy-butyrate increased significantly in the group anaesthetized with the lower dose of fentanyl. In the group that received high-dose fentanyl anaesthesia the plasma concentrations of almost all the hormones and substances measured remained relatively low. The differences between the two groups during surgery were significant for adrenaline (P less than 0.001) and cortisol (P less than 0.001). High-dose fentanyl appears to block the trauma-induced stress response seen in patients anaesthetized with low dose fentanyl.