The beneficial effects of estrogen on vasculature are partially explained by an estrogen-induced increase in nitric oxide (NO) synthesis. Asymmetric dimethylarginine (ADMA) is an endogenous competitive inhibitor of NO synthase. In the present study, the effect of 17 beta-estradiol (E2) on ADMA and NO synthesis was investigated both in vivo and in vitro. Plasma NO and ADMA levels were measured in healthy women at a low menstrual estrogenic stage (E2 < 100 pg/ml) and in women who were undergoing ovarian hyperstimulation (E2 < 2000 pg/ml) before in vitro fertilization embryo transfer. Primary human umbilical vein endothelial cell (HUVEC) cultures were incubated with and without 100 nM E2 for 24 hours and the NO and ADMA levels of the media were measured. A nitric oxide analyzer was used for the detection of NO metabolites. ADMA and L-arginine were measured by high-performance liquid chromatography after precolumn derivatization with o-phthaldialdehyde. The IVF patients with high plasma E2 concentrations had significantly lower (48%, n = 12) plasma ADMA and higher (56%, n = 14) NO levels than the women at a low estrogenic stage. The incubation of HUVEC cultures with estradiol resulted in a significant decrease (47%, n = 10) in ADMA and an increase (46%, n = 10) in NO concentration in the culture media. Estradiol, by reducing ADMA, may therefore facilitate NO synthesis in endothelial cells. The protective effects of estradiol on vasculature may partly be related to a reduction in ADMA levels.