Abstract Objective—To evaluate the antinociceptive effects of epidurally administered hydromorphone in conscious, healthy cats. Animals—7 healthy adult cats. Procedures—An epidural catheter was implanted in each cat. Thermal threshold (TT) was measured by increasing the temperature of a probe placed on the thorax and monitoring the cat's response. Mechanical threshold (MT) was measured by manually inflating a modified blood-pressure bladder affixed to a thoracic limb and monitoring the response. After the baseline TT and MT values were determined, hydromorphone (0.05 mg/kg) or an equal volume of saline (0.9% NaCl) solution was epidurally injected. The TT and MT were again measured at 15, 30, 45, 60, 120, 180, 240, 300, 360, and 480 minutes after injection. Results—TT and MT did not change significantly from baseline values at any point after saline solution was administered. The MT and TT values were significantly higher than the baseline value at 15 minutes and at 120 and 180 minutes after hydromorphone administration, respectively. The MT and TT values after hydromorphone administration were also significantly different from those obtained at 30 minutes and at 15 minutes and 120 to 300 minutes, respectively, after administration of saline solution. No significant changes in skin temperature were detected after either treatment. Conclusions and Clinical Relevance—Epidural administration of hydromorphone at a dosage of 0.05 mg/kg yielded thermal and some mechanical antinociceptive effects in cats, and no hyperthermia was detected. Additional studies of the antinociceptive effectiveness and duration of epidurally administered hydromorphone in clinical situations are required.