A canine phaeochromocytoma was diagnosed by (1) argyrophil methods, (2) immunoreactivity to chromogranin and to vasoactive intestinal peptide (VIP), and (3) the demonstration of typical electron-dense granules by transmission electron microscopy. This is the first report of immunoreactivity to general neuroendocrine markers such as chromogranin, and to VIP, in a phaeochromocytoma in domestic animals. The use of these markers for the differential diagnosis of phaeochromocytoma, and the role of VIP in the severe chronic diarrhoea shown by the dog in this study are discussed.