In 28 children undergoing adenoidectomy, plasma concentrations of catecholamines, ACTH and cortisol were measured. Fourteen children were anaesthetized with halothane (seven non-intubated, seven intubated) and 14 with enflurane (seven non-intubated, seven intubated). During undisturbed anaesthesia, plasma catecholamines were significantly higher with halothane than with enflurane (P less than 0.05). Immediately after surgery, catecholamines were increased up to 300% in the halothane groups. In the enflurane groups, however, the catecholamine concentrations remained unchanged. This difference between the two agents, after surgery, was statistically significant (P less than 0.01 for intubated and P less than 0.001 for non-intubated children). Fifteen minutes postoperatively no difference was found in plasma concentrations between the groups. In all four groups, plasma concentrations of ACTH and cortisol increased similarly during the procedure. It was concluded that plasma catecholamines were higher during halothane than during enflurane anaesthesia in children undergoing adenoidectomy. This difference may be caused by a stimulating effect of halothane on the endogenous catecholamine release. This increased sympathomimetic response during halothane anaesthesia was correlated to the incidence of ventricular arrhythmias previously found with this agent during adenoidectomy.