Visceral mast cell tumors (MCT) are rare, malignant, highly aggressives and usually associated with extremely poor prognosis. A 4 year old male mixed breed dog was referred for an endoscopic evaluation of hematochezia and large bowel diarrhea. Physical examination was unremarkable. Colonscopy revealed a mass at about 17 cm. from the anus. Histopathology revealed a severe inflammatory infiltrate with eosinophils. Due to recurrence of intestinal signs and enlargement of the previously reduced mass after discontinuation of therapy with prednisone, surgery was performed. The mass with the regional lymph node was resected. Histology on the surgical piece revealed an intestinal MCT. Endoscopic biopsies were unable to detect the neoplasm. Age of the dog, primary site of the mass, surgery outcome and survival time are different from what usually reported as the biological behavior of visceral MCT in literature.