Testosterone may have contributing role in endothelial dysfunction. A part of testosterone that diffuses to the interior of endothelial cells will be converted to estradiol by aromatase enzyme. It is known that endothelial cells contain estrogen receptors (ER) as well as androgen receptor (AR). In the development of cardiovascular diseases, ER has been regarded as a protective factor. To examine the influence of testosterone to the expression of endothelial ER-β, we have exposed testosterone in 4 doses: 0 nM, 1 nM, 10 nM, and 100 nM to the endothelial cells culture derived from human umbilical vein (HUVEC) in either normoglucose (NG) or high glucose (HG) medium. HG medium was used to mimic hyperglycemia, a condition that can trigger activation of endothelial cells. Immunocytochemistry was used to stain the endothelial ER-β. The results showed that proportion of endothelial cells positively stained with ER-β antibody was significantly higher in NG medium + testosterone than in NG medium only. However, proportion of endothelial cells positively stained with ER-β antibody in HG medium + testosterone did not significantly differ than in HG medium only. In conclusion, testosterone increases the expression of beta estrogen receptors in resting endothelial cells but not in activated endothelial cells.