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Objective—To compare time of onset and duration of analgesia obtained by epidural administration of a lidocaine–magnesium sulfate (MgSO4) combination versus a lidocaine–distilled water combination in cattle. Animals—5 healthy adult (mean ± SD age, 4.6 ± 0.5 years) Holstein cows (body weight, 480.0 ± 71 kg). Procedures—Caudal epidural anesthesia was obtained in all cows with 2% lidocaine (0.22 mg/kg) solution and 1 mL of distilled water, and 2 weeks later, anesthesia was repeated with 2% lidocaine (0.22 mg/kg) solution and 1 mL of 10% MgSO4 solution. Time to onset, duration, and cranial spread of analgesia were recorded. Heart rate, respiratory rate, and body temperature were recorded at minute 0 prior to epidural administrations of each treatment (baseline) and at 5, 10, 15, 30, 60, and 75 minutes after epidural administration of each treatment. Results—Onset of analgesia was significantly different following epidural administration of lidocaine–distilled water (mean ± SD, 2.82 ± 0.33 minutes) versus lidocaine-MgSO4 (4.76 ± 0.59 minutes). Epidural administration of lidocaine-MgSO4 produced analgesia of significantly longer duration (168 ± 2.6 minutes) than that of lidocaine–distilled water (59.8 ± 3.4 minutes). No significant differences were observed in heart rate, respiratory rate, and body temperature, compared with baseline values, for lidocaine–distilled water or lidocaine-MgSO4 treatments. Conclusions and Clinical Relevance—The combination of lidocaine with MgSO4 produced analgesia of longer duration than lidocaine with distilled water. By use of this combination, long-duration obstetric and surgical procedures could commence relatively soon after epidural injection and could be completed without readministration of anesthetic.