This study was performed to compare the onset and duration of analgesia produced by either a lidocaine� MgSO4 or lidocainedistilled water combination ad- ministration in the caudal epidural space of horse. Seven healthy adult horses, aged 11.7 � 1.4 years (mean � SD), body weight (kg) 567 � 32.5 (meanSD), were selected for this study. Caudal epidural anesthesia was produced in all horses by administering 2% lidocaine (0.22 mg/kg) diluted in 1 mL distilled water and repeated with 2% lidocaine (0.22 mg/kg) diluted in 1 mL 10% MgSO4 2 weeks later. Time to onset (minutes), duration (minutes), and cranial spread of epi- dural analgesia were recorded. Heart rate (HR), respira- tory rate (RR), and body temperature (8C) were recorded. Measurements were taken at 0 (as a baseline value before epidural administrations) and at 5, 10, 15, 30, 60, and 75 minutesafter the epidural administrations of each treatment. Statistical analyses included paired Student t test and analysis of variance (computer pro- gram SPSS, Analytical Software, version 15.00). Statisti- cal significance was set at P < .05. Onset of analgesia was significantly different (P < .001) between lidocaine-dis- tilled water (2.38 � 0.47 minutes) and lidocai- neMgSO4 (4.62 � 0.54 minutes). Duration of analgesia after lidocaineMgSO4 (186.0 � 7.0 minutes) was longer than lidocaine-distilled water (54.5 � 7.3 minutes). No significant differences were recorded for HR, RR, and body temperature in compar- ison with baseline values for each group. Using the lidocaineMgSO4 combination for obstetric and surgi- cal procedures could commence relatively soon after epidural injection and could be completed without readministration of anesthetic agent.