Spontaneous myxomatous mitral valve disease (MMVD) in dogs has common features with primary mitral valve prolapse in people. Altered valvular motion and changed haemodynamic forces due to mitral valve prolapse and mitral regurgitation might influence the valvular tissue. Endothelial cells exposed to shear stress in vitro exhibit synthesis and release of mediators that can interact with the subendothelial tissue. Interestingly, associations have been found between MMVD disease severity and local valvular expression of endothelin receptors, nitric oxide (NO) synthase, serotonin-receptors and extracellular matrix signalling molecules in dogs. Plasma markers of endothelial function are changed in dogs with MMVD. Plasma markers of NO bioavailability are associated with disease severity. In addition, plasma von Willebrand factor is decreased in dogs with MMVD. The plasma markers are influenced by several factors such as dog breed, exercise and clinical settings and therefore highly standardised study conditions are required (Moesgaard et al. 2007). An increased platelet aggregation response has been found in Cavalier King Charles Spaniels, a dog breed highly predisposed to MMVD. Changes in platelet function might be associated with development of the disease. However, recent studies concerning relations between MMVD and platelet function have shown that the results depend much on dog breed and the technique used to assess platelet function. In conclusion, expression of local valvular mediators is altered in dogs with MMVD. Moreover, endothelial and platelet function are changed. It is important to be aware of factors (e.g. breed differences) that may influence results when evaluating especially endothelial and platelet function. Key reference: Moesgaard, S.G., Holte, A.V., Mogensen, T., Mølbak, J., Kristensen, A.T., Jensen, A.L., Teerlink, T., Reynolds, A.J., Olsen, L.H. 2007. Res Vet Sci 82:409–15.