Prolonged hypokinesia caused cardiac deadaptation in rabbits which resulted in diminishing of the left ventricle speed of contraction and relaxation joined later by a decreased vascular resistance. As a result, the ejection velocity as well as the stroke volume and the cardiac output were normal. The decrease of the relaxation speed became more obvious at a high heart rate and resulted in shortening of the diastolic pause and diminishing of cardiac output. Hearts of the hypokinetic animals were characterized by normal maximal pressure developed by a unit of myocardial mass at aorta clamping, decrease adrenoreactivity, and increased cholinoreactivity. This complex of changes is contrary to changes observed in adaptation to exercise, but is similar to changes observed in the compensatory hypertrophy of the heart.