In previous studies we have reported that intact female rats fed a high phytoestrogen diet were protected against adverse left ventricular (LV) remodeling induced by chronic volume overload. We hypothesized that both phytoestrogens and ovarian hormones, particularly estrogen, were necessary for this dietary-induced cardioprotection. To test this hypothesis, eight groups of female rats were studied, which were fed either a high phytoestrogen (+phyto) or phytoestrogen-free diet, including sham-operated rats (Sham), intact rats with fistula (Fist), ovariectomized rats with fistula (Fist-OX) and Fist-OX rats treated with estrogen (EST). Myocardial function and remodeling were assessed after eight weeks of volume overload using a blood-perfused isolated heart apparatus. Fist-OX rats developed significant ventricular dilatation and increased compliance versus intact Fist rats, which were associated with a significant decrease in contractility. Estrogen treatment prevented pulmonary edema and attenuated LV hypertrophy and dilatation, but did not maintain contractility. However, dietary phytoestrogens completely prevented LV dilatation in both the Fist+phyto and Fist-OX+EST+phyto groups, but had no effect on LV remodeling in the Fist-OX+phyto group. Contractility was significantly greater in the estrogen treated rats fed the phytoestrogen diet, versus those treated with estrogen alone. Dietary phytoestrogens did not affect LV or uterine mass, serum estrogen, LV estrogen receptor expression or cardiac function in Sham animals. These data indicate that estrogen is not solely responsible for the cardioprotection exhibited by intact females, and that phytoestrogens can work synergistically with ovarian hormones to attenuate ventricular remodeling induced by chronic volume overload in female rats.