BACKGROUND: The purpose of this study was to evaluate the preemptive analgesic effect of a single intravenous infusion of low dose ketamine-clonidine. We compared the placebo with low dose ketamine and low dose ketamine-clonidine in patients undergoing an appendectomy. METHODS: Sixty ASA class 1 or 2 patients who received general anesthesia for an appendectomy were allocated randomly to three groups. Group 1 received intravenous normal saline 6 ml and group 2 received ketamine 0.15 mg/kg in normal saline 6 ml and group 3 received ketamine 0.15 mg/kg and clonidine 1microgram/kg in normal saline 6 ml 5 minutes before surgical incision. In the recovery room postoperative analgesia was assessed by the verbal numerical rating scale (VNRS) at 0 min, 15 min, 30 min, 60 min, 90 min and 120 min. Vital signs, sedation score and side effects were also checked. For postoperative analgesia, morphine 2 mg was given intravenously whenever a patient complained of pain, or the VNRS score was above 7. RESULTS: There were no intergroup differences of individual morphine requirements for postoperative pain control among the three groups. VNRS score, blood pressure, heart rate, sedation score and side effects were not significantly different among the three groups. CONCLUSIONS: A single low dose of ketamine-clonidine may not produce a postoperative analgesic effect in the recovery room, and a single low dose of ketamine also may not produce the preemptive effect for at least 2 hours.