A medullary carcinoma of the thyroid in a dog first became evident because of a distant metastasis. The histologic characteristics of the tumor differ from those previously reported in dogs in that this tumor was highly malignant; anaplasia and mitotic figures were common. The diagnosis was substantiated by radioimmunoassay for calcitonin. Serotonin and 5-hydroxytryptophan also were elevated in the neoplastic tissue. The relation of the biochemical alterations to the clinical course was not clear.