We report the modulatory effects of chronic oral LY139481 (raloxifene) on basal release of nitric oxide (NO) and mRNA levels of endothelial NO synthase (eNOS) in rat thoracic aorta. Constrictor dose-response curves to phenylephrine were generated before and after pretreatment with N(omega)-nitro-L-arginine methyl ester (L-NAME), an inhibitor of NO synthase. Aortic segments were obtained from four groups of rats gavaged orally for 21 days: (i) ovariectomized, (ii) sham, (iii) ovariectomized estradiol-treated, and (iv) ovariectomized raloxifene-treated. Intact aortic rings from sham rats and ovariectomized rats receiving raloxifene and estrogen showed a greater potentiation of the phenylephrine responses after L-NAME. Semi-quantitative reverse transcription-polymerase chain reaction indicated a gender-based difference in eNOS mRNA expression in thoracic aorta. Moreover, we demonstrated that eNOS mRNA expression in the upper thoracic aorta was significantly higher in treatment groups. These results show that chronically administered raloxifene is exerting a potentially important vasculo-protective effect by stimulating eNOS expression.