The aim of this randomized, double blind clinical trial study was to compare the effects of ketamine and paracetamol for postoperative pain management after total abdominal hysterectomy. After receiving approval from Ethical Committee of Tehran University of Medical Science and written informed consent, we enrolled 80 patients of ASA physical status I-II scheduled for elective total abdominal hysterectomy. Patients with a history of addiction to opiates, known allergy to acetaminophen or ketamine, alcoholism and drug abuse, hepatic disease, sizure and aged below 25 yr and above 65 yr were excluded from the study. patients were assigned to one of two treatment groups according to a computer-generated list of random numbers. Patients in group I, 40 patients, received 1 gr intravenous acetaminophen (Apotel) intraoperatively until skin closure. whereas those in goup II,40 patients , received intravenous bolus ketamine 0.15 mg/kg after extubation. A patient control analgesia device with 15 cc fentanyl and 85 cc normal salin was used to provide postoperative pain relief. Each ml of solution contained 7.5 mcg of fentanyl. The solution infused continuously 4 ml per hour for first 24 hour after surgery. Pain Score, Sedation Score, Blood pressure and heart rate, Nausea and Vomiting and use of additional analgesia evaluated at the PACU and 4, 6, 12, 24 hours after surgery. Pain assessed by Visual Analogue Score (0= no pain, 10= the worst imaginable pain). The demographic characteristics of the two groups data of study recorded on common questionarie form and data sheet. After that, data analysis planed after completion of the trial.