12/15-Lipoxygenase (12/15-LOX) plays a pathogenic role in atherosclerosis. To characterize whether 12/15-LOX also contributes to endothelial dysfunction and hypertension, regulation of vessel tone and angiotensin II (ang II) responses were characterized in mice deficient in 12/15-LOX. There was a twofold increase in the magnitude of l-nitroarginine-methyl ester-inhibitable, acetylcholine-dependent relaxation or phenylephrine-dependent constriction in aortic rings isolated from 12/15-LOX(-/-) mice. Plasma NO metabolites and aortic endothelial NO synthase (eNOS) expression were also elevated twofold. Angiotensin II failed to vasoconstrict 12/15-LOX(-/-) aortic rings in the absence of L-nitroarginine-methyl ester, and ang II impaired acetylcholine-induced relaxation in wild-type, but not 12/15-LOX(-/-) rings. In vivo, 12/15-LOX(-/-) mice had similar basal systolic blood pressure measurements to wild type, however, blood pressure elevations in response to ang II infusion (1.1 mg/kg/day) were significantly attenuated (maximal pressure, 143.4 +/- 4 mmHg versus 122.1 +/- 5.3 mmHg for wild type and 12/15-LOX(-/-), respectively). In contrast, vascular hypertrophic responses to ang II, and ang II type 1 receptor (AT1-R) expression were similar in both strains. This study shows that 12/15-LOX(-/-) mice have increased NO biosynthesis and impaired ang II-dependent vascular responses in vitro and in vivo, suggesting that 12/15-LOX signaling contributes to impaired NO bioactivity in vascular disease in vivo.