Nitric oxide (NO), an important vasodilatory modulator of systemic and pulmonary vascular tone, is synthesized from L-arginine by the enzyme NO synthase in vascular endothelial and smooth muscle cells. L-Arginine analogs, such as N omega-nitro-L-arginine methyl ester (L-NAME), are competitive antagonists of NO synthase and inhibit NO synthesis. Group B streptococcus (GBS) causes pulmonary hypertension, hypoxemia, lung vascular injury, and reduced cardiac output in both human newborns and neonatal piglets. Lung vascular injury associated with prolonged GBS infusion in piglets may attenuate NO production and thus promote severe pulmonary hypertension. We studied the effect of the NOS inhibitor, L-NAME and the precursor of NO, L-arginine, on pulmonary and systemic hemodynamics during late-phase GBS sepsis in the piglet model. Neonatal piglets were anesthetized, ventilated with room air, and randomized to receive a continuous infusion of saline (n = 5) or GBS (n = 5) for 4 h. After 3 h of infusion, both groups received a bolus of L-NAME (3 mg/kg). Hemodynamic and gas exchange indices were measured at baseline, 30 min, and 3 h of infusion, and 30 min and 1 h after L-NAME treatment. L-NAME treatment caused 1) significant increases in mean pulmonary arterial pressure, pulmonary vascular resistance, mean systemic arterial pressure, and systemic vascular resistance for both groups; 2) a similar percentage of increase in pulmonary vascular resistance for the two groups; 3) greater reduction in cardiac output and SV in the GBS compared with the control group; and 4) no significant alterations in arterial partial pressure of oxygen or the difference between alveolar and arterial partial pressure of oxygen for either group.(ABSTRACT TRUNCATED AT 250 WORDS)