A 10-year-old female neutered boxer was presented with a five-week history of episodic collapse and melaena. Twenty-four-hour electrocardiograph (Holter) analysis revealed the collapsing episodes to coincide with episodes of paroxysmal ventricular tachycardia. Investigation of the dog's melaena revealed a gastric ulcer which was treated medically and an ileocaecal mass which was surgically excised. Histopathological examination of the mass was consistent with a neuroendocrine (carcinoid) tumour. The patient's recovery after surgery was unremarkable. At six-week follow-up, there had been no further episodes of melaena or collapse and repeat Holter analysis did not show any significant abnormalities. In this dog the gastric ulceration and paroxysmal ventricular tachycardia could be attributed to a paraneoplastic syndrome as a result of circulating vasoactive substances released by the tumour; this is supported by the evidence that all clinical signs resolved after surgical excision of the mass and the dog was clinically well 18 months after surgery.