Abstract Objective—To evaluate the effects of epidural administration of hydromorphone on avoidance threshold to noxious electrical stimulation of the perineal, sacral, lumbar, and thoracic regions in horses. Animals—6 healthy adult horses. Procedure—Horses were assigned to receive hydromorphone (0.04 mg/kg) or a control solution (20 mL of sterile water) administered epidurally into in the first intercoccygeal space. Treatments were administered at time intervals of ≥ 7 days. Electrical stimulation was applied for 6 hours after epidural injection over the dermatomes of the perineal, sacral, lumbar, and thoracic regions, and the avoidance threshold voltage was recorded. Results—Administration of sterile water did not change the avoidance threshold. Hydromorphone significantly increased the avoidance threshold by 20 minutes after injection, which lasted until 250 minutes after epidural administration in the perineal, sacral, lumbar, and thoracic regions. Profound analgesia (avoidance threshold > 40 V) was achieved only in the perineal region at 60 minutes after epidural administration of hydromorphone. Analgesia for all dermatomes was considered moderate for 250 minutes after epidural injection. Conclusions and Clinical Relevance—Epidural administration of hydromorphone increases the avoidance threshold to noxious electrical stimulation in the perineal, lumbar, sacral, and thoracic regions in horses for 250 minutes after injection. Hydromorphone epidural administration may prove useful in the management of horses with pain of moderate to mild intensity.