In the pathogenesis of the formation and progression of chronic heart failure syndrome in dogs with myxomatous degeneration of the mitral valve an important role is played by the chronic increase in the concentration of serum proinflammatory cytokines. The purpose of the work is to study the concentration of proinflammatory cytokines in the blood serum of dogs with myxomatous degeneration depending on the degree of functional class of heart failure, presence of pulmonary edema, left atrial dilatation, and cardiogenic cachexia. The blood serum in 40 dogs with myxomatous degeneration of the mitral valve revealed a higher concentration of interleukin-1α, interleukin-6, interleukin-8 and tumor necrosis factor-α compared to 15 clinically healthy animals. Serum concentration of these proinflammatory cytokines was found to be particularly high in the development of pulmonary edema, left-side atrial remodeling and cardiogenic cachexia syndrome, as well as it positively correlates with the functional class of chronic heart failure and the size of the left atrium. Activation of proinflammatory cytokines plays an important role in the pathogenesis of formation and progression of myxomatous degeneration of the mitral valve in dogs.