We tested the hypothesis that hyperoxia does not cause adequate constriction of choroidal vessels of the newborn (1 to 5 days old) pig, resulting in increased O₂ delivery to the retina, possibly due to excess production and/or effects of vasodilators such as nitric oxide (NO). Hyperoxia (100% O₂, 45 minutes) led to a decrease in retinal blood flow (RBF) of both newborn and juvenile (5 to 6 weeks old) pigs and also reduced choroidal blood flow (ChBF) in juvenile but not in newborn pigs; the absence of hyperoxia-induced ChBF response in the newborn was associated with a rise in choroidal O₂ delivery. Ibuprofen (prostaglandin G/H synthase inhibitor) and 1,3-dimethyl-2-thiourea (a free radical scavenger) did not modify the choroidal hemodynamic responses to hyperoxia in newborn pigs. However, in newborn animals treated with the NO synthase (NOS) inhibitor N^G-nitro-l-arginine methyl ester (L-NAME), hyperoxia caused a decrease in blood flow and O₂ delivery to the choroid. Consistent with these effects of L-NAME, hyperoxia induced an increase in choroidal cGMP in newborn pigs ventilated with 100% O₂ and stimulated nitrite production in isolated choroids exposed to hyperoxia from newborn but not juvenile pigs; these effects were inhibited by NOS blockers. Also, both constitutive and inducible NOS activities were higher in choroidal tissues from newborn than from juvenile animals. In addition, the vasorelaxant effect of the NO donor sodium nitroprusside in vitro was also greater on choroids from newborn than from juvenile pigs. Finally, L-NAME prevented the hyperoxia-induced increase in peroxidation products in the choroid of newborns. It is concluded that hyperoxia does not lead to a decrease in blood flow and O₂ delivery to the choroid of the newborn because of increased NO synthesis and effects; since the choroid is the main source of O₂ supply to the retina, the present data contribute in providing an explanation for the increased susceptibility of the immature neonate to hyperoxia-induced retinopathy.