In a randomised controlled study in 16 orthopaedic patients, the influence of midazolam-fentanyl-N2O/O2 anesthesia (group A) resp. halothane-N2O/O2 anesthesia (group B) on the plasma concentrations of the endocrine parameters ACTH, aldosterone, cortisol, 17-DHEA, insulin, prolactin, T3, T4, TBG (thyroxine bounded globuline) as well as adrenaline, noradrenaline, and dopamine was investigated. Additionally the metabolites glucose, lactate, free glycerin, and acetacetate were measured. Beside prolactin values, only the values for ACTH, aldosterone, cortisol, and 17-DHEA differed with respect to both anesthesia methods. Under halothane-N2O/O2 anesthesia free T4 rose initially also, here represented by T4/TBG-ratio (= FTI). However, the fall of T3 concentration showed no phase - resp. anesthesia-specific changes. Catecholamine levels reached highest values towards the end of operation resp. one hour after extubation in both groups. The insulin secretion, however, was not significantly raised in either group during acute stress phases. As an expression of modified metabolic regulation comparable rises of plasma levels of glucose, lactate, free glycerin, and acetacetate were observed under midazolam-fentanyl-N2O/O2 anesthesia as well as under halothane-N2O/O2. According to presented data, both methods of anesthesia modulated the endocrine metabolic response of the organism to surgical stress, without showing any clinically relevant advantages or disadvantages attributable to either method.