Background Cardiovascular diseases (CVD) are less prevalent in postmenopausal women received estrogen replacement therapy (ERT) than those who did not receive ERT.Previous study has shown that the increase of nitric oxide (NO) synthesis is one of the cardioprotective effects of estrogen.This study investigated the effects of estrogen and L-arginine (L-Arg) on serum NO concentrations and the possible regulatory role in endothelial nitric oxide synthase (eNOS) expression in aortas of aged rats.Methods Fifty aged female wistar rats (18-20 months) were randomly divided into five groups (n=10):Sham group (sham operated,0.9 % NaCl 10 μg every three day for 4 months),OVX group (ovariectomized,0.9 % NaCl 10 μg i.m every three day for 4 months),OVE group (ovariectomized + 17β-estradiol 10 μg i.m every three day for 4 months),OVE + L-Arg group (ovariectomized + 17β-estradiol 10ug i.m every three day + 2.25 % L-Arg contained in drinking water every day for 4 months) and L-Arg group (ovariectomized + 2.25 % L-Arg contained in drinking water every day for 4 months).NO concentration and the expression of eNOS mRNA in aorta were measured after 4 months.Results Serum NO synthesis did not alter after ovariectomized (P=0.362),but were increased in OVE group,L-Arg group and OVE + L-Arg group compared with OVX group (P < 0.05,P < 0.05,P < 0.01,respectively).NO concentration also increased in OVE + L-Arg group when compared with OVE group (P < 0.05) or L-Arg group (P < 0.05).There was no significant difference in eNOS mRNA expression in aortas of aged rats between sham,OVX,OVE,OVE + L-Arg and L-Arg group (F=0.550,P=0.700).Conclusions Estrogen treatment and L-Arg supplementation increase serum NO synthesis,but do not upregulate eNOS mRNA expression in aortas of aged rats. Background Cardiovascular diseases (CVD) are less prevalent in postmenopausal women received estrogen replacement therapy (ERT) than those who did not receive ERT. Previous studies have shown that the increase of nitric oxide (NO) synthesis is one of the cardioprotective effects of estrogen. This study investigated the effects of estrogen and L-arginine (L-Arg) on ​​serum NO concentrations and the possible regulatory role in endothelial nitric oxide synthase (eNOS) expression in aortas of aged rats. Methods Fifty aged female wistar rats (18-20 Months were randomly divided into five groups (n = 10): Sham group (sham operated, 0.9% NaCl 10 μg every three days for 4 months), OVX group (ovariectomized, 0.9% NaCl 10 μg im every three days for 4 months ), OVE group (ovariectomized + 17β-estradiol 10 μg im every three days for 4 months), OVE + L-Arg group (ovariectomized + 17β-estradiol 10ug im every three days + 2.25% L-Arg contained in drinking water every day for 4 months) and L-Arg group (ovariect omized + 2.25% L-Arg contained in drinking water every day for 4 months) .NO concentration and the expression of eNOS mRNA in aorta were measured after 4 months. Results Serum NO synthesis did not alter after ovariectomized (P = 0.362), but were also increased in OVE group, L-Arg group and OVE + L-Arg group compared with OVX group (P <0.05, P <0.01, respectively) with OVE group (P <0.05) or L-Arg group (P <0.05). There was no significant difference in eNOS mRNA expression in aortas of aged rats between sham, OVX, OVE, OVE + L-Arg and L-Arg group (F = 0.550, P = 0.700) .Conclusions Estrogen treatment and L-Arg supplementation increase serum NO synthesis, but do not upregulate eNOS mRNA expression in aortas of aged rats.