In this case, a female boxer dog, 6-year-old, with tonic-clonic convulsions and hind-limb paresis was evaluated. The serum insulin and serum glucose levels were 58 mu U/ml and 5 mg/dl, respectively. In spite of all care, the comatose dog died. At necropsy, one well-circumscribed white-grayish mass, 4x3x2 cm, was found at the caudal edge of right lobe of the pancreas. Subcortical malacic areas in reddish color was seen in the brain. Microscopically, the tumoral mass consisted of neoplastic cells with round shape, basophilic nucleous and granular eosinophilic cytoplasm. In addition to these findings, metastasis to the regional lymph node of the pancreas was observed. Immunohistochemically, the tumor cells from both primary and metastatic tissues showed immunoreactivity to monoclonal mouse anti-insulin antibody. Clinically, histologically and immunohistochemically, functional insulinoma with lymph node metastasis was diagnosed in the present case.