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Estradiol increases dendritic spine density and synaptogenesis in the CA1 region of the female hippocampus. This effect is specific to females, as estradiol-treated males fail to show increases in hippocampal spine density. Estradiol-induced spinogenesis in the female is dependent upon upregulation of the N-methyl-D-aspartic acid (NMDA) receptor as well as on non-nuclear estrogen receptors (ER), including those found in dendrites. Thus, in the male, the inability of estradiol to induce spinogenesis may be related to a failure of estradiol to increase hippocampal NMDA receptors as well as a paucity of dendritic ER. In the first experiment, we sought to investigate this possibility by assessing NMDA receptor binding, using [3H]-glutamate autoradiography, in estradiol-treated males and females. We found that while estradiol increases NMDA binding in gonadectomized females, estradiol fails to modulate NMDA binding in gonadectomized males. To further investigate sex differences in the hippocampus, we conducted a second separate, but related, ultrastructural study in which we quantified ERα-immunoreactivity (ERα-ir) in neuronal profiles in the CA1 region of the hippocampus in intact males and females in diestrus and proestrus. Consistent with previous reports in the female, we found ERα-ir in several extranuclear sites including dendrites, spines, terminals and axons. Statistical analyses revealed that females in proestrus had a 114.3% increase in ERα-labeled dendritic spines compared to females in diestrus and intact males. Taken together, these studies suggest that both the ability of estrogen to increase NMDA binding in the hippocampus and the presence of ERα in dendritic spines may contribute to the observed sex difference in estradiol-induced hippocampal spinogenesis.