Background and Objective: The aim of this study was to evaluate the effect of the addition of magnesium to prilocaine on the duration of sensory and motor-nerve block. Methods: After institutional approval, 60 ASA physical status I and II patients, between 18 and 60 years of age and scheduled for forearm and hand surgery under axillary brachial plexus block, were included in the study. Patients were randomly assigned to 1 of 4 groups. All of the patients received 5 mg/kg of 2% prilocaine and isotonic sodium chloride solution in 35 mL total volume for axillary brachial plexus block. Group I received intravenous saline, and group II received 150 mg intravenous magnesium at the same time as local anesthetic administration. In group III, 100 mg of magnesium, and in group IV, 150 mg of magnesium, were added to local anesthetic solution. Sensory block and motor block of musculocutaneous, radial, median, and ulnar nerves were recorded at 5-minute intervals. Results: The duration of motor block was significantly longer in group IV than in other groups (167 ± 30, 177 ± 17, 180 ± 20, and 250 ± 19 minutes in groups I to IV, respectively) (P < .01). Mean duration of sensory block in both of the perineural magnesium groups was statistically different than in groups I and II (P < .001). Duration of sensory block in group IV (304 ± 30 minutes) was significantly longer than in group III (253 ± 23 minutes) (P < .001). Conclusion: The admixture of magnesium to prilocaine for axillary brachial plexus block provided a pronounced prolongation of sensory and motor block without side effects. © 2006 American Society of Regional Anesthesia and Pain Medicine.