A 7-year-old male castrated Boxer dog was presented to the referring veterinarian for an acute onset of regurgitation and vomiting. After a preliminary diagnostic evaluation did not yield a diagnosis, endoscopy was performed, disclosing severe esophagitis, pyloric mucosal hypertrophy, and multifocal duodenal ulcerations. Plasma gastrin concentration was increased at 276 ng/L (10-40 ng/L). These findings were supportive for a diagnosis of gastrinoma. Buster was referred to the Cornell University Hospital for Animals for nuclear scintigraphy using 111Indium-pentetreotide to confirm the diagnosis of gastrinoma, approximate its anatomical location, and rule out metastates. This report reviews the etiology, diagnosis, clinical course, and treatment of hypergastrinemia, specifically canine gastrinoma.