Objective To investigate histological and ultrastructural changes in myocardium and liver from rats chronically exposed to an altitude of 3 418 m. Methods 20 Wistar rats (male and female each half) were transported to the altitude from an area of 5m above sea level within 6 days. 8 weeks later, the heart and liver tissues of the rats were got and observed under light and electron microscope and the number of RBC and concentration of Hb measured before and after exposure to the altitude. Results The main histological changes in cardiac and liver tissues were cell swelling ,that is, myocardial granular degeneration and liver hydrophobic change. Other lesions were myocytoplasm condensation, liver acidophilic degeneration and small necrotic foci within myocardial fiber, without necroses observed in liver tissue. The main ultrastructural changes included dilation of myoplasmic reticulum, swelling of mitochondria and a decrease in glucogen granule. No irreversible damage was observed such as mitochondria with bar-like,three-membrane or high-density-membrane crista and margination of chromatin. There were protrusions extending to capillary cavity from endothelial cells which also had cytoplasmic swelling and fewer pinosome. In addition, the number of RBC and concentration of Hb increased significantly. Conclusions Myocardial and liver alterations of the rats exposed to a high altitude (3 418 m) for 8 weeks were reversible. Left ventricle alteration was not significantly different from that of the right, and the liver lesions were less severe than those in myocardium.