The nitric oxide and cyclic GMP production in myocardium early after burn injury in rats were investi- gated. Nitric oxide synthase activity was measured in cytosols from the left ventricular wall of burned rats. Cytosols from the control group animals were shown to contain mainly Ca

2+

dependent nitric oxide synthase （cNOS） with small amount of Ca

2+

independent nitric oxide synthase （iNOS）. Following burn injury

there was a marked increase in iNOS activity with a peak at sh post-burn

however

myocardial cNOS ac- tivity was found to decline obviously. Parallel to iNOS induction there was a significant increase in myocar- dial nitric oxide and cyclic GMP production. All these changes were alleviated by treatment of the rats with dexamethasone. Since increases in cyclic GMP levels in the heart were associated with reduced myocardial contractility

it is possible that enhanced production of nitric oxide by a Ca

2+

independent NO synthase ac- counts

at least in part

for the depression of myocardial contractility seen in burn animals and patients.