<正> Objective. To investigate the role of endogenous carbon monoxide （CO） in hypoxia.Methods. After rats were inhaled with hypoxic gases and the heme oxygenase inhibitor ZnPPIX was administered

we measured the CO levels in plasma

liver

lung and kidney. Meanwhile plasma cGMP levels were observed. Furthermore

we recorded the changes of hemodynamic and blood gases.Results. Acute mild hypoxia （10% O2） significantly increased CO levels in plasma as well as liver

kidney and lung

while acute severe hypoxia （5% O2） significantly decreased CO levels in plasma as well as liver

kidney and lung. In addition

the former significantly elevated cGMP levels in plasma while the latter markedly reduced cGMP levels in plasma. The hemodynamic changes occurred in accordance with the changes of carbon monoxide.Conclusions. Our results indicate

for the first time

that the endogenous carbon monoxide plays an important role in regulating the vessel tone during hypoxia.