Angiotensin II (ANG II) produces vasoconstriction by a direct action on smooth muscle cells via AT₁ receptors. These receptors are also present in the endothelium, but their function is poorly understood. This study was therefore undertaken to determine whether ANG II elicits the release of nitric oxide (NO) from cultured rat aortic endothelial cells. NO production, measured by the accumulation of nitrite and nitrate, was enhanced by 10⁻⁷ M ANG II. The biological activity of the NO released by ANG II action was evaluated by measuring its guanylate cyclase-stimulating activity in smooth muscle cells. The guanosine 3′,5′-cyclic monophosphate (cGMP) content of smooth muscle cells was significantly increased by exposure of supernatant from ANG II-stimulated endothelial cells. These effects resulted from the activation of NO synthase, as they were inhibited by the l-arginine analogs. These ANG II actions were mediated by the AT₁ receptor, as shown by their inhibition by the AT₁ antagonist losartan. The cGMP production by reporter cells was inhibited by the calmodulin antagonist W-7, suggesting that ANG II activates endothelial calmodulin-dependent NO synthase. This hypothesis is also supported by the increase of intracellular free calcium induced by ANG II in endothelial cells. ANG II also stimulated luminol-enhanced chemiluminescence in endothelial cells. This effect was inhibited byN ^ω-monomethyl-l-arginine and superoxide dismutase, suggesting that this luminol-enhanced chemiluminescence reflected an increase in peroxynitrite production. Thus ANG II stimulates NO release from macrovascular endothelium, which may modulate the direct vasoconstrictor effect of ANG II on smooth muscle cells. However, this beneficial effect may be counteracted by the simultaneous production of peroxynitrite, which could contribute to several pathological processes in the vascular wall.