The experimental results are described confirming the hypothesis that the viscous properties of the passive cardiac muscle are connected with the extracellular fluid filtration in the elastic medium formed by the connective tissue network and myocytes. It is shown that the relaxation properties are more pronounced in cold-blooded animals myocardium (frog, turtle) than in that of warm-blooded (cat, rabbit), which correlates with the smaller connective tissue content and larger porosity of myocardium in cold-blooded animals. The decrease in porosity of the cardiac muscle samples by reducing the osmosis of the surrounding solution or squeezing the fluid out of the samples by mechanical torsion results in the slowing down of the stress relaxation, the increase in porosity of the muscle in the hyperosmotic solution results in its speeding up. The increase of the surrounding solution viscosity by adding saccharose to it leads to the proportional stress relaxation deceleration, which agrees quantitatively and qualitatively with the advanced hypothesis.