The endocrine stress response under inhalation anesthesia with halothane, enflurane, and isoflurane was investigated in 30 patients during and after orthopedic surgery (Table 2). Plasma levels of adrenaline and noradrenaline (by HPLC/ECD), ADH, ACTH, and cortisol (by RIA), glucose, lactate, and free glycerol were determined before induction of anesthesia, 10 min after intubation, 10 min before the end of the operation, and 5 and 30 min after extubation. Statistical evaluation was undertaken by analysis of variance with repeated measures on one factor. P values of less than 0.05 were considered significant. There were no significant differences in the concentrations of plasma catecholamines (Table 4, Figs. 1 and 2), ADH, ACTH (Table 5, Figs. 3 and 4), or cortisol before and during surgery between the groups. ADH was lower in the halothane group 5 and 30 min after extubation (P = 0.05), which might be due to the prolonged elimination of halothane after anesthesia. Blood pressure, heart rate (Table 3), and plasma concentrations of glucose, lactate, and free glycerol (Table 6) were comparable in all groups. It is concluded that for clinical practice halothane, enflurane, and isoflurane are comparable in their influence on the surgical stress response.