Nitric oxide formation and cyclic GMP level in the myocardium were studied in the early stage after radiation, bum andcombined radiation-bum injuries in rats. Nitric oxide synthase (NOS) activity was measured in the cytosol of the left ventricularwall. In the controls, the cytosol was found to contain mainly Ca2+ -dependent NOS (cNOS) and a small amount of Ca2+ -inden-pendent NOS (iNOS). After burn and combined radiation-burn injuries, a marked increase of iNOS activity with a peak in the 8thhour postinjury was found but the myocardial cNOS activity declined obviously. Parallel to iNOS activity increase, there was a significant increase of myocardial production of NO and cGMP. The combined effcts of radiation and burn injuries on the rats weremore severe than those of burn injury alone. All the changes could be prevented by the administration of dexamethasone. No obvious changes were observed in the rats after radiation injury alone. Since the increase of cGMP level in the heart is associated withreduced contractility, it is possible that the increased production of NO stimulated by iNOS accounts at least partially, for the depression of myocardial contractility after bum and combined radiation burn injury. Nitric oxide formation and cyclic GMP level in the myocardium were studied in the early stage after radiation, bum andcombined radiation-bum injuries in rats. Nitric oxide synthase (NOS) activity was measured in the cytosol of the left ventricular wall. In the controls, the After burn and combined radiation-burn injuries, a marked increase of iNOS activity with a peak in the 8thhour postinjury was found but the myocardial cNOS activity declined obviously. Parallel to iNOS activity increase, there was a significant increase of myocardial production of NO and cGMP. The combined effcts of radiation and burn injuries on the rats were more severe than those of burn injury. All the changes could be prevented by the administration of dexamethasone. No obvious changes were observed in the rats after radiation injury alone. Since the increase of cGMP level in the heart is ass ociated withreduced contractility, it is possible that the increased production of NO stimulated by iNOS accounts at least partially for the depression of myocardial contractility after bum and combined radiation burn injury.