Among the identified risks and benefits of hormone-replacement therapy, the effects of treatment on cognitive function in postmenopausal women have proved difficult to define. Here we conducted a controlled, prospective analysis in a nonhuman primate model to test whether surgical menopause and estrogen replacement influence the cognitive outcome of normal aging. Sixteen aged rhesus monkeys were ovariectomized, and throughout the course of subsequent neuropsychological assessment, half received a regimen of low-dose, cyclic estradiol replacement. Hormone treatment substantially reversed the marked age-related impairment vehicle-injected monkeys exhibited on a delayed response test of spatial working memory. Modest improvement was also observed on a delayed nonmatching-to-sample recognition memory task. In contrast, ovariectomy exacerbated age-related deficits in object discrimination learning; the magnitude of this effect was equivalent among vehicle- and estrogen-treated monkeys. Together, these results demonstrate that ovarian hormone status can broadly influence normal cognitive aging in monkeys, affecting capacities mediated by multiple brain regions, including the prefrontal cortex and the medial temporal lobe memory system. The animal model established here should enable progress toward defining the neurobiological mechanisms that mediate the beneficial effects of estrogen on age-related cognitive decline in primates.