The effect of two different oxygen inspiratory fractions (FiO2 = 21% and 100%) on the hemodynamic responses induced by N omega-nitro-L-arginine methyl ester (L-NAME) was investigated in anesthetized dogs. L-NAME (0.01-10.0 mg/kg), but not D-NAME, induced dose-dependent changes in the hemodynamic parameters of the animals. At the highest dose, L-NAME increased mean arterial blood pressure in both room air (from 86.2 +/- 3.2 to 125.1 +/- 7.8 mmHg) and pure oxygen (from 100.0 +/- 7.5 to 139.0 +/- 3.2 mmHg) ventilated animals. L-NAME also increased systemic and pulmonary vascular resistances. These effects were accompanied by a decrease in cardiac output and bradycardia (37% and 31% decreases for pure oxygen and room air, respectively). However, there were no significant differences in the responses to L-NAME between the dogs ventilated with FiO2 = 21% and those ventilated with FiO2 = 100%. L-NAME did not modify blood gas analyses, despite the expected difference in pO2 levels between the two experimental groups of animals (3 times higher in the animals ventilated with pure oxygen). These results indicate that nitric oxide release accounts for the maintenance of hemodynamic function in the anesthetized dog, and that L-NAME-induced effects are not affected by hyperoxemia.2933-9