Aim: Treatment with ketamine and meperidine is effective in postoperative shivering. The aim of this study was to investigate the minimum effective dose of ketamine in the prevention of postanaesthetic shivering compared to placebo and meperidine. Material and Method: This prospective randomized double-blind study involved 150 ASA I and II patients undergoing general anesthesia. Patients were randomly allocated to receive normal saline (Group S, n = 30), meperidine 20 mg (Group M, n = 30), ketamine 0.1 mg/kg (Group K1, n = 30), ketamine 0.25 mg/kg (Group K2, n = 30) and ketamine 0.5 mg/kg (Group K5, n = 30) intravenously 20 minutes before completion of surgery. The anesthesia was induced with propofol 2 mg/kg, fentanyl 1 aeg/kg and vecuronium 0.1 mg/kg. It was maintained with sevoflurane 2-3% and nitrous oxide 60% in oxygen. Tympanic temperature was measured immediately after induction of anesthesia, 30 minutes after induction and before administration of the study drug. Postoperative shivering was recorded using a four point scale and postoperative pain using a visual analogue scale (VAS) ranging between 0 and 10. Results: The number of patients shivering on arrival the recovery room and at 10 minutes after operation were significantly less in Groups M and K5 than in Groups K1, K2 and Group S (p < 0.001 and p = 0.001). The time to first analgesic requirement in Groups M and K5 was longer than in the Groups K1, K2 and Group S (P < 0.001). There was no difference between the five groups regarding VAS pain scores (p > 0.05). Discussion: Prophylactic usage of ketamine 0.5 mg/kg was effective to prevent postanaesthetic shivering, but ketamine 0.1 mg/kg and 0.25 mg/kg had no prophylactic effect.