By histological, morphometric and gravimetric methods the hearts of 7 monkeys exposed to head-down tilt at -6 degrees were examined (2 monkeys were exposed to head-down tilt for 7 days and 5 monkeys were exposed to hypokinesia for 7 days and then to head-down tilt for 12 days; 4 additional monkeys were used as controls). During head-down tilt the heart weight increased due to blood pooling in the Thebesian vessels which was in turn associated with blood redistribution and accumulation in the vessels and parenchymal organs of the upper body. During 7- and 12-day head-down tilt myocardiocytes of the papillary muscles of the ventricles were not enlarged, and the number of functioning capillaries in the papillary muscles diminished.