The incidence of cardiovascular disease is markedly lower in cycling, pre-menopausal women and post-menopausal women receiving estrogen than in men or untreated post-menopausal women. Clinical studies demonstrate a protective role of estrogen in hormone replacement therapy in terms of reducing cardiovascular risk. However, the benefits of hormone replacement therapy in cardiovascular disease remain unclear. We investigated the effects of estrogen on the contractile responses of the renal artery of ovariectomized Wistar rats (OVX) compared to both ovariectomized 17β-estradiol-treated rats (OVXE) and sham-operated (control) rats. Isometric contraction of renal artery was recorded with a strain gauge transducer. The maximum contractile response of the renal artery smooth muscle to KCl (80 mM) in the OVXE group was significantly higher than that in both the control and OVX groups. The phenylephrine (PE) concentration-response curves in all three groups indicated a greater sensitivity at lower concentrations of PE following treatment with 100 μM L-arginine methyl ester (L-NAME). The EC₅₀ values for PE in the three groups were 2 times lower in the presence of L-NAME than those lacking exposure to L-NAME. The EC₅₀ value for PE in the OVX group was ~3 times lower in the presence of L-NAME than in those lacking exposure to L-NAME and 100 nM BMY 7378, an α_1D-adrenoceptor antagonist. The rate of relaxation of the PE-induced contraction (T_1/2) was significantly reduced in the OVX group relative to both the control and OVXE groups. T_1/2 values after treatment with 100 μM L-NAME were slower than those lacking exposure to L-NAME in all groups. Further, the T_1/2 value of the OVX group was 2 times greater than that of the control; this change was reversed in the OVXE group. In conclusion, our results suggest that estrogen regulates contraction and relaxation in the renal artery via NO synthase activity and alteration of the Ca²⁺ transport systems.