Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide (NO) synthase produced by endothelial cells. ADMA levels are mainly regulated by the activity of dimethylarginine dimethylaminohydrolases (DDAH-I and DDAH-II). Endothelial release of ADMA is increased in the presence of oxidized LDL cholesterol (oxLDL), whereas estrogens stimulate endothelial NO production by increasing NO synthase. These results have been performed in endothelial cells from venous origin, mainly in cultured human umbilical vein endothelial cells (HUVEC). Our aim was to compare the estradiol effects on changes induced by oxLDL on DDAH expression and ADMA production in cultured human umbilical artery endothelial cells (HUAEC) with results obtained in HUVEC. Primary HUAEC and HUVEC were exposed to 100 µg/mL of oxLDL, with or without 1 nM estradiol for 24 hours. ADMA was measured in culture supernatants by HPLC. DDAH-I and II mRNA expression and protein content was quantified by real time PCR and immunoblotting, respectively. ADMA production remained unaltered in HUAEC exposed to estradiol, whereas in HUVEC estradiol decreased ADMA production by 57% (p<0.001). In both cell types, oxLDL significantly increased ADMA production by ~50% (p<0.01). Finally, combined exposure to oxLDL plus estradiol completely abolished the increased production of ADMA induced by oxLDL (p<0.05 vs. oxLDL alone). DDAH-I protein expression remained unchanged after all treatments. Exposure of HUVEC to oxLDL, reduced DDAH-II expression by 20% (p<0.01) at both the mRNA as well as the protein levels, which in turn increased ADMA levels. Estradiol alone increased DDAH-II mRNA and protein expression up to 170 % (p<0.01). In cells exposed to estradiol in combination with oxLDL, DDAH-II protein levels were the same as those for estradiol alone. Nevertheless, in HUAEC the effects were quite different. Estradiol alone did not modify DDAH-II expression, whereas oxLDL increased DDAH-II expression by 50% (p<0.05), probably as a consequence of increased ADMA levels. To check that possibility, DDAH activity was measured. DDAH activity was reduced by oxLDL by 50 % (p<0.05). Estradiol alone did not modify DDAH activity, but restored the reduced activity caused by oxLDL (p<0.01 vs. oxLDL), thus resulting in decreased ADMA levels. Therefore, estradiol counteracts oxLDL-induced ADMA production in HUVEC, whereas in HUAEC, estradiol counteracts the raise of ADMA levels and the reduced DDAH activity induced by oxLDL, whereas the increased DDAH-II expression seems to be a consequence of increased ADMA levels.