Alterations in endothelium-derived relaxing factor (EDRF) production or mechanism of action may be involved in the responses of the developing pulmonary vasculature to changes in oxygenation. In this study the effects of acute changes in in vitro oxygen tension on EDRF production were determined in isolated segments of ovine fetal intrapulmonary arteries (4th generation) obtained at 125-135 days of gestation (term 144 +/- 4 days). EDRF production was assessed by measuring segment guanosine 3',5'-cyclic monophosphate (cGMP) accumulation in the presence of a phosphodiesterase inhibitor. Basal (nonstimulated) cGMP production and cGMP production with acetylcholine (ACh) stimulation were dependent on the presence of the endothelium, on the availability of L-arginine, and on soluble guanylate cyclase activity, confirming that they were indicative of EDRF production. cGMP production with sodium nitroprusside (SNP) was used to discriminate changes in the sensitivity of soluble guanylate cyclase with varying conditions. With decreasing oxygen tension, basal and ACh-stimulated cGMP production were attenuated, whereas cGMP production with SNP was not, indicating oxygen modulation of EDRF production. Studies of endothelium-dependent relaxation yielded comparable findings in that the response to ACh was attenuated, but that to SNP was not altered by decreased oxygenation. In addition, the decline in endothelium-dependent relaxation with decreased oxygen tension was rapidly reversed when oxygenation was increased. Parallel experiments examining cGMP production in similarly sized mesenteric arteries revealed that the effect of oxygen on pulmonary artery EDRF production may be specific to that vascular bed. These findings indicate that oxygen selectively modulates EDRF production and endothelium-dependent relaxation in ovine fetal pulmonary arteries. Direct effects of oxygen on EDRF production may at least partially underlie the responses of the developing pulmonary circulation to changes in oxygenation.