Antenatal glucocorticoid therapy improves pulmonary function in preterm newborns. We have determined the effect of antenatal glucocorticoid therapy on nitric oxide-mediated relaxation in pulmonary vessels of preterm lambs. Ovine fetuses (126 days gestation; full term = 150 days) were injected with betamethasone (0.5 mg/kg body wt) or saline. After 48 h, lambs were delivered, ventilated for 3 h, and killed. Isolated fourth-generation pulmonary arteries (2-3 mm diameter) and veins (1.5-2 mm diameter) were suspended in organ chambers filled with modified Krebs-Ringer solution (95% O2-5% CO2) at 37 degrees C, and their isometric tension was recorded. During contractions to endothelin-1 or U-46619 (in the presence of indomethacin), acetylcholine and bradykinin induced endothelium-dependent nitro-L-arginine-inhibitable relaxation in arteries and veins. The relaxation was greater in veins of betamethasone-treated than in those of control lambs. Veins from lambs without endothelium treated with betamethasone were more sensitive to sodium nitroprusside than veins from controls. For arteries, there was no significant difference in relaxation between different groups. Relaxation induced by 8-bromoguanosine 3',5'-cyclic monophosphate was similar in arteries and veins of different groups. Radioimmunoassay showed that nitric oxide caused a greater increase in guanosine 3',5'-cyclic monophosphate in betamethasone-treated veins than in controls. These data suggest that antenatal betamethasone therapy augments nitric oxide-mediated relaxation of pulmonary veins of preterm lambs, probably by increasing soluble guanylate cyclase activity of vascular smooth muscle.