Normotensive female rats exhibit age-related decreases in estrous cyclicity and increases in blood pressure. In spontaneously hypertensive rats, estrogens, including dietary phytoestrogens, prevent or attenuate the increased blood pressure associated with estrogen depletion. The present studies examine the effects of ovariectomy (OVX) at either 3 or 10 mo of age. Although blood pressure increases from 3 to 9 mo, OVX at 3 mo of age has no added effect - despite the fact that OVX (compared to ovary-intact) rats weighed significantly more. In contrast, aging from 10 to 16 mo is associated with a further increase in blood pressure, which is potentiated by estrogen depletion. Removal of dietary phytoestrogens exacerbated the hypertensive effects of OVX in these middle-aged rats. As in younger rats, estrogen depletion at 10 mo of age was associated with greater weight gain. Whereas estrogen depletion at 3 mo of age was without effect on fluid intake over the next 6 mo, OVX at 10 mo of age was associated with decreased fluid intake. In a final study, rats were OVX at 3 mo of age with estradiol (E2) treatment initiated at 10 mo of age. Long-term OVX (>10 mo) was associated with increased blood pressure and mortality at 14-16 mo of age. Circulating levels of E2 were decreased by OVX. Plasma aldosterone was increased by OVX, an effect which was prevented by either E2 or phytoestrogens. Neither E2 nor aldosterone was affected by age. These data indicate that (a) the physiological effects of estrogen depletion vary with age; (b) phytoestrogens in the diet exert some protective effects; and (c) long-term OVX in the absence of hormone replacement is associated with premature mortality. We suggest that chronic increases in aldosterone and sympathetic tone underlie the hypertensive effects of estrogen depletion. © 2004 by Humana Press Inc. All rights of any nature whatsoever reserved.