Abstract.A cholecystectomy was performed on a 10-year-old spayed female mixed-breed dog with chronic weight loss, persistently increased liver enzyme activities, and cholecystomegaly identified by ultrasonographic examination. A subsequent diagnosis of a biliary carcinoid was made based on a neuroendocrine-type histologic pattern, cytoplasmic argyrophilia by Grimelius staining, immunopositivity for chromogranin A, and the ultrastructural finding of cytoplasmic secretory granules in neoplastic cells. Extrahepatic biliary carcinoid tumors are rare tumors of humans and have not been documented in domestic animals.Key words: Dogs; gallbladder; biliary; carcinoid. In humans and animals, carcinoid tumors are uncommon neoplasms that arise from dispersed cells of the neuoroendocrine system in the gastrointestinal tract, biliary system, pancreas, and lung. These cells belong to one of two functional groups: 1) the amine precursor uptake and decarboxylation cells that produce serotonin and adrenocorticotrophic hormone, or 2) those cells capable of synthesizing low molecular weight polypeptide or protein hormones such as chromogranin, cholecystokinin and secretin. Definitive diagnosis of carcinoid tumors is based on histologic features, immunostaining for secretory products such as chromogranin, and ultrastructural identification of neuroendocrine secretory granules. 2 In dogs, hepatic, gastrointestinal, and pulmonary carcinoids have been reported. [5][6][7] Intestinal carcinoids have also been documented in a horse and a cow. 2 In horses, three cases of maxillary sinus carcinoid tumors have been reported as well. 8 In humans, carcinoid tumors are frequently found in the gastrointestinal system (74%) and the bronchopulmonary system (25%). Within the gastrointestinal tract, the most common sites include the small bowel (29%), appendix (19%), and rectum (13%). 3 In this case, a 10-year-old spayed female mixed-breed dog initially presented for weight loss and persistent vomition. No abnormalities were noted on physical examination. Serum biochemistry abnormalities included increased liver enzyme activities, including aspartate aminotransferase (625 U/ L; reference range 10-60 U/L), alanine aminotransferase (2,225 U/L; reference range 8-57 U/L), lactate dehydrogenase (531 U/L; reference range (0-250 U/L), and alkaline phosphatase (4,460 U/L; reference range 10-140 U/L) as well as hyperbilirubinemia (Total bilirubin 5.6 mg/dL; reference range 0-0.6 mg/dL) consistent with hepatocellular damage and cholestasis. During the next 8 weeks, progres-