T he mechanisms by which hypoxia causes vasoconstriction in vivo are not known. Accumulating evidence implicates the endothelium as a key regulator of vascular tone. Hypoxia induces the expression and secretion of endothelin-1 (ET-1), a potent vasoconstrictor in cultured human endothelial cells. We report here that nitric oxide (NO), an endothelial-derived relaxing factor, modifies this induction of ET-1. Whereas low oxygen tension (PO2 = 20-30 Torr) increases ET-1 expression four- to eightfold above that seen at normal oxygen tension (PO2 = 150 Torr), sodium nitroprusside, which releases NO, suppresses this effect. This inhibition of hypoxia-induced ET-1 expression occurs within the first hour of exposure of cells to sodium nitroprusside. Moreover, when the endogenous constitutive levels of NO made by endothelial cells are suppressed using N-omega-nitro-L-arginine, a potent competitive inhibitor of NO synthase, the baseline levels of ET-1 produced in normoxic environments are increased three- to fourfold. The effects of hypoxia and the NO synthase inhibitor on ET-1 expression are additive. The regulation of ET-1 production by NO appears to be at the level of transcription. Similar effects of NO were observed on the expression of the PDGF-B chain gene. PDGF-B expression was suppressed by NO in a hypoxic environment and induced by N-omega-nitro-L-arginine in both normoxic and hypoxic environments. These findings suggest that in addition to its role as a vasodilator, NO may also influence vascular tone via the regulated reciprocal production of ET-1 and PDGF-B in the vasculature.