The mitochondriom of left-ventricular cardiomyocytes in rats subjected to head-down tilt was studied at the electron microscopic level. The ultrastructure of individual mitochondria was disturbed and the number of intermitochondrial contacts in the perinuclear zone increased after 30-day head-down tilt. These parameters did not return to normal over 30 days after 30-day tilt. Repeated 14-day head-down tilt led to a more pronounced increase in the number of intermitochondrial contacts, mitochondria with abnormal ultrastructure were still observed.