To determine whether angiotensin (ANG) II, a vasoconstrictor hormone, activates constitutive nitric oxide synthase (cNOS) in endothelial cells (ECs), we investigated the cellular mechanism by which ANG II induces nitric oxide (NO) formation in cultured bovine ECs. ANG II rapidly (within 1min) and dose-dependently (10^-9-10^-6M) increased nitrate/nitrite (NOx) production. This effect of ANG II was abolished by a NOS inhibitor, N^G-monomethyl-L-arginine. An ANG II type 1 (AT₁) receptor antagonist (DuP 753), but not an ANG II type 2 (AT₂)receptor antagonist (PD 123177), dose-dependently inhibited ANG II-induced NOx production. A Ca²⁺-channel blocker (barnidipine) failed to affect ANG II-induced NOx production, whereas an intracellular Ca²⁺ chelator (BAPTA) and a calmodulin inhibitor (W-7) abolished NOx production induced by ANG II. A protein kinase C (PKC) inhibitor (H-7) and down-regulation of endogenous PKC after pretreatment with phorbol ester decreased NOx production stimulated by ANG II. ANG II transiently stimulated inositol 1, 4, 5-trisphosphate (IP₃) formation, and increased cytosolic free Ca²⁺ concentrations; these effects were blocked by DuP 753. Our data demonstrate that ANG II stimulates NO release by activation of Ca²⁺/calmodulin-dependent cNOS via AT₁ receptors in bovine ECs. (Hypertens Res 1996; 19: 201-206)