Background and aims: During induction of anesthesia, when we are waiting to initiate the effect of neuromuscular blocking drugs, the patients suffer risk of aspiration. If the onset of neuromuscular action was more rapid, the risk would decrease. In order to reach this goal, we can do different things, such as use a priming dose of neuromuscular blocking drugs before major dose. Till now, in previous researchers, this has never done.Materials & methods: A randomized double blind study was designed on 88 ASA 1 & 2 patients who were candidate for obstetrics & general surgeries. Patients were randomly assigned to three groups. Different groups received normal saline, 25mg/kg magnesium sulfate and 50 mg/kg magnesium sulfate slowly & via a micro set during ten10 minutes. Blood pressure and heart rate were measured. Then, iv midazolam and fentanyl were administered as premedication. We calculated total dose of cisatracurium and 10% of this dose was administered as priming dose. After 3 minutes, thiopental sodium and remaining dose of cisatracurium were given. Neuromuscular monitoring with a nerve stimulator and single twitch test was done until the twitches were disappeared. The times were measured in all groups with a chronometer. Then, the patients were intubated and anesthesia was maintained with prpopfol. Heart rate and blood pressure were measured again. 2, 5, 10, 15, 20, 25 and 30 minutes after intubation, intensity of blockade by PTC test, heart rate and blood pressures were measured.Results: The onset of neuromuscular blockade in group 3 was faster than the normal saline group. The mean time of muscle relaxation in group 1, 2 and 3 were 209, 226 and 188 seconds respectively. Intensity of neuromuscular blocking by PTC test in group 3 was more than group 2 & in latter was more than normal saline group. Arrhythmia were seen in 6 patients from all cases & both belonged to group 3. Side effects incidence of magnesium sulfate injection were more common in group 3.Conclusion: Using magnesium sulfate before cisatracurium during induction of anesthesia with priming method leads to increase onset and intensity of neuromuscular blockade of cisatracurium. Alterations of blood pressure and heart rate were constant after injection of magnesium.