We reported that estrogen treatment of ovariectomized rats increased uterine smooth muscle contractility and the C-terminal myosin heavy chain isoform SM1:SM2 ratio (19). We extended this model to study gender and estrogen effects on vascular contractility. Experimental groups included 10-14 wk male (M), female (F), ovariectomized (OF) and OF treated with estrogen (OF&E) for 7 days with a subcutaneous pellet delivery system, resulting in 17-estradiol of 85 (OF&E) vs 5 pg/ml (OF or M). The SM1:SM2 ratio increased from 1.8 to 2.6 in thoracic aorta, similar to uterine muscle. Isometric force was measured in 5 mm segments of intact and endothelium-denuded (-endo) aorta. KCl: maximum forces, OF M > OF&E; and ED50, OF&E > OF M. Differences in ED50 with estrogen persisted after endothelial denudation. The decreased force in -endo OF aorta was not seen in OF&E, suggesting that estrogen altered an endothelium-dependent effect. Norepinephrine: no differences in maximum forces; and ED₅₀, OF > OF&E > M. Estrogen treatment, in contrast to KCl, increased sensitivity. Endothelial denudation increased sensitivity but reduced the differences between groups. Acetylcholine relaxation: males were more sensitive than females and estrogen had no effect. In the abdominal aorta, there were no changes in SM1:SM2 with 17-estradiol and differences in contractility were blunted. In summary, estrogen treatment decreased responses to KCl but increased sensitivity to norepinephrine; male rats always demonstrated the highest contractility. An increase in the C-terminal myosin heavy chain isoform SM1:SM2 ratio with 17-estradiol treatment may underlie the changes observed in contractility.