A case report is given of a very rare spontaneous mast cell tumor in the eyelid of the left eye of a female Wistar rat used in a long-term oral toxicity study. Metastasis of the tumor had occurred in the mandibular lymph nodes and in the liver. Clinically, the animal showed blepharospasm, dacryorrhoea, and exophthalmus. Hematologic findings included slight eosinophilia and a remarkable basophilia. At necropsy, a bilateral conjunctivitis was diagnosed and a tumorous mass was found in the left submandibular region. Histologically, the tumor was composed of round to polygonal cells with pale cytoplasm, containing abundant predominantly basophilic granules. The intracytoplasmatic granules stained metachromatically with Toluidine blue and immunostained positively with serotonin. Numerous eosinophils were scattered throughout the tumor and were also present in other organs. Cells with round, oval, or indented nuclei and abundant cytoplasm, containing pronounced eosinophilic granules, were found in spleen and bone marrow. They turned out to he immature stages of eosinophilic granulocytes. Characteristics of the present tumor are compared with observations on mast cell tumors in other species.