This prospective, double-blind, randomized study assessed effect of pre-emptive peritonsillar block in 30 ASA–I children, aged 6–12 years, of both sexes, scheduled for tonsillectomy. Patients were divided into three groups: those in group I received a sham block, whereas peritonsillar blocks with bupivacaine 0.25% were given to the children before tonsillectomy (group II) or immediately after surgery had been completed (group III). Constant pain, pain on swallowing, blood glucose, serum epinephrine and norepinephrine concentrations were measured immediately after surgery and 4 h after operation. Patients in group I experienced more pain (P < 0.05) than those in groups II and III, both in the immediate postoperative period and over the next 4 h. Patients in groups II and III experienced similar pain (P > 0.05). The pain experienced when water was swallowed was similar to that of the constant pain. The rise of serum norepinephrine concentration in group II was significantly less (P < 0.05) compared to groups I and III. We found both pre-emptive and postoperative block to be equally effective in treating postoperative pain, with pre-emptive block being more effective in preventing the rise in norepinephrine concentration.