An 11-year-old female crossbred dog showed signs of polyuria, polydipsia, vomiting, posterior weakness and ataxia. Clinical and laboratory findings suggested the diagnosis of polycythaemia vera. The haematological values shown over a six-month period are presented. In four samples some aspects of erythrocyte function (glucose-6-phosphate dehydrogenase [G6PD] and pyruvate kinase [PK] activities, 2,3 diphosphoglycerate [2,3 DPG] concentration, osomotic fragility and intracellular sodium and potassium concentrations) were studied. Variable activities of G6PD and PK, probably related to different reticulocyte number, were detected together with normal osmotic fragility and intracellular sodium and potassium concentrations. 2,3 DPG concentration was higher than normal in all four samples. This could be interpreted as a response to a low tissue perfusion rather than a higher content of 2,3 DPG in red blood cells from the polycythaemic dog.