Neonatal pulmonary hypertension is treated with pure oxygen and nitric oxide (NO) as a vasodilating agent. Since NO is a radical, known to induce oxidative effects by producing peroxynitrite and since treatment with increased oxygen levels leads to higher occurence of reactive oxygen species these two types of treatment have high risks for oxidative side-effects. In order to observe this we instrumented five newborn piglets (age 8-10 days, weight 2.2-3.8 kg) under general anaesthesia and curarization and mechanical ventilation. Two hours after surgery while ventilated with normal air the animals were exposed sequentially to inhalation of NO (80 ppm) and NO plus pure oxygen for one hour each. Carotid artery blood samples from the were drawn hourly (4 ml) for analyses of thiobarbituric acid reactive substances(TBArs) and every 20 minutes (2.5 ml) for total (GT) and oxidized (GSSG) glutathione and glutathione S-transferase rho (GST). TBArs levels, indicative of lipid peroxidation, did not increase, in fact a small decrease (p=0.10; two tailed linear regression) of 7.4 nanomoles/l per hour was found, which was probably the result of blood sampling. GT values in red cells did not change. Red blood cell GST, marker for oxidative stress, did not change. Whole blood GSSG (n=4) showed tendency to increase (p=0.30) but this increase was not related to NO or oxygen. In fact three of the four experiments showed a rise in GSSG values starting immediately after instrumentation with a maximum after less than two hours (i.e., before NO treatment). These results showed that early effects of oxidative stress may have developed, possibly caused by the instrumentation. The main conclusion of this study is that inhalation of NO or oxygen did not produce detectable oxidative stress in blood.