The discovery of tissue protective effects of erythropoietin has stimulated significant interest in erythropoietin (Epo) as a novel therapeutic approach to vascular protection. The present study was designed to determine the cerebral vascular effects of recombinant Epo in vivo. Recombinant adenoviral vectors (10⁹ pfu/animal) encoding genes for human erythropoietin (AdEpo) and beta-galactosidase (AdLacZ) were injected into the cisterna magna of rabbits. After forty-eight hours, basilar arteries were harvested for analysis of vasomotor function, Western blotting and measurement of cGMP levels. Gene transfer of AdEpo increased the expressions of recombinant Epo and its receptor in the basilar arteries. Arteries exposed to recombinant Epo demonstrated attenuation of contractile responses to histamine (10^-9 to 10^-5 mol/L) (P<0.05, n=5). Endothelium-dependent relaxations to acetylcholine (10^-9 mol/L to 10^-5 mol/L) were significantly augmented (P<0.05, n=5), while endothelium-independent relaxations to a NO-donor, DEA-NONOate remained unchanged in AdEpo-transduced basilar arteries. Transduction with AdEpo increased the protein expression of endothelial nitric oxide synthase (eNOS) and phosphorylated (S1177) form of the enzyme. Basal levels of cyclic GMP were significantly elevated in arteries transduced with AdEpo consistent with increased NO production. Our studies suggest that in cerebral circulation, Epo enhances endothelium-dependent vasodilatation mediated by nitric oxide. This effect could play an important role in the vascular protective effect of Epo.