An 8-year-old, male, castrated, medium-sized domestic rabbit presented with a bilateral, transient and stress-related exophthalmos. Thoracic radiographs revealed a large mass cranial to the heart. The rabbit was euthanized because of severe respiratory distress. At necropsy, a large well-circumscribed, kidneyshaped mass was present in the cranial mediastinum without macroscopic abnormalities in the other organs.The mass itself was composed of a mixed population of small and medium sized round cells in a fine reticular stroma. The medium sized cells and large cells stained intensely positive for cytokeratin in the cytoplasm, confirming the diagnosis of thymoma and excluding lymphoma.