The myocardial ultrastructural responses to simulated microgravity were investigated in rats subjected to tail-suspension for 40 or 120 days. Ultrastructural changes observed in the myocardium of tail-suspended animals included intra-and extra-mitochondrial glycogen accumulation, and extensive capillary endothelial projections. These morphological alterations are qualitatively similar to those of the rats carried by biosatellites. In addition, a storage of lipofuscin was found in the myocardial cells in 120-day tail-suspended animals. The results suggest a depressed glycolysis and an enhanced degradation in the myocardium and supported the hypothesis that the heart during long-term simulated microgravity was in a hypodynamic state.