Background:Postoperative pain control (especially, after adenotonsillectomy) has a very important effect on recovery time, hospitalization duration, hemodynamic disorders, bleeding, nausea, vomiting and medical costs.Objectives:The aim of this study was to investigate and compare the effects of intravenous and rectal acetaminophen on controlling post-adenotonsillectomy pain in children, and duration of their analgesic effects.Patients and Methods:In this randomized double-blinded clinical trial, 96 children aged 4 - 10 years old with ASA physical status I or II who were candidates for adenotonsillectomy surgery in Amir-al-Momenin Hospital, Rasht, Iran were entered into the study and randomly divided into two equal groups. Anesthesia in both groups was induced injecting fentanyl-thiopental and at racurium; afterwards is of lurane was used to maintain anesthesia. After anesthesia induction, one group received intravenous and the other one, rectal acetaminophen, and were later compared based on CHIPPS criteria.Results:Data analysis indicated a significant relationship between reduction of postoperative pain and the use of intravenous or rectal acetaminophen (P = 0.0001); in group receiving IV acetaminophen, only 10.4% of patients had no pain whereas in group receiving rectal acetaminophen, this number reached 43.8%. Also, on 4 and 6 hour time intervals, pain in rectal acetaminophen receiving group was less than that in IV acetaminophen receiving group (P < 0.05). Demand for additional analgesic medication in rectal acetaminophen receiving group was less than that in IV group (P = 0.0001).Conclusions:Post-operative pain in rectal acetaminophen group was less than that in intravenous acetaminophen group, and rectal acetaminophen group demanded their first additional analgesic medication later.