A 14 year old male castrated English Springer Spaniel presented to the Cornell University Hospital for Animals with episodic collapse of one month's duration. Abdominocentesis revealed the presence of septic peritonitis. Radiographic and ultrasonographic studies revealed that gastrointestinal perforation was the likely cause of peritonitis. Exploratory celiotomy revealed jejunal ulcerationand perforation. Surgical resection and anastomosis of the affected area was performed. Recovery from surgery was uneventful, and clinical signs resolved. Histopathologic examination of the resected jejunum revealed chronic inflammatory and ulcerative changes, with extension of the inflammation into the associated mesentery. Possible causes of jejunal ulceration are discussed, and non-steroidal anti-inflammatory drug (deracoxib) induced intestinal injury is proposed as the most likely cause in this case.