Effects of oral administration for 4 weeks of a soy fraction with mainly isoflavones (Glycine max, Leguminosae) (SOYPH; 5 mg/kg) on vascular dysfunction induced by bilateral ovariectomy (OVX) in rats were studied. We evaluated vascular reactivity of aortic rings after acetylcholine (ACh 10 nM-10 microM), sodium nitroprussiate (SN 15 - 30 nM) and NG-L-arginine (L-NMA; 10 - 100 microM). Uterine weight and nitric oxide synthase (NOS) activity were also investigated. The same parameters were observed after 4 weeks treatment with 17beta-estradiol. In OVX rats endothelial-dependent vascular responses were changed: reduction of induced contraction ( L-NMA 100 mM: sham OVX 2.1 +/- 0.2 g/mg tissue; OVX 1.7 +/- 0.4 g/mg tissue). Ovariectomy produced a reduction of constitutive NOS activity. Uterine weight was increased in animals treated with 17beta-estradiol but not with SOYPH. Either SOYPH or 17beta-estradiol produced a similar improvement of endothelial dysfunction and increased NOS activity. Our data suggest that soy isoflavones produce an improvement of endothelial dysfunction induced by ovariectomy so as 17beta-estradiol, but probably without changes in reproductive system.