Background <p>Myxomatous mitral valve disease is the most common cardiac disease among dogs. The mitral valve consists of valve leaflets, chordae tendineae and papillary muscles. Much attention has been given to lesions of the valve leaflets, but there is still a lack of information in the literature about the chordae. During myxomatous mitral valve disease, an episode of chordae rupture can occur. Predisposing factors for such an episode are known in human cardiology.</p> Materials and methods <p>The 54 mitral valves were prepared from the hearts of older dogs, from which the chordae tendineae were then isolated. The chordae tendineae were subjected to two tests: biomechanical and histopathological. The first consisted of a uniaxial tensile test to determine the mechanical strength values of chordae tendineae. The histopathological examination was based on a 4-point scale (0–3 scale), which assessed the extent of degenerative changes within chordae tendineae. The biomechanical and histopathological findings were correlated in the statistical analysis.</p> Results <p>Degenerated chordae tendineae tended to rupture more rapidly during the static stretching test as compared to healthy chordae. The more advanced the degenerative changes in the chordae tendineae, the more the biomechanical function was affected.</p> Conclusions <p>Degeneration affects the biomechanical properties of the mitral valve chordae tendineae in dogs, as confirmed by chordae tendineae stress studies.</p>