Dendritic spines in the hippocampus are sources of synaptic contact that may be involved in processes of learning and memory [Moser (1999) Cell. Mol. Life Sci., 55, 593-600]. These structures are sensitive to sex differences as females in proestrus possess a greater density than males and females in other stages of the estrous cycle [Woolley et al. (1990) J. Neurosci., 10, 4035-4039]. Moreover, exposure to an acute stressful event increases spine density in the male hippocampus but decreases spine density in the female hippocampus [Shors et al. (2001) J. Neurosci., 21, 6292-6297]. Here we demonstrate that antagonism of N-methyl-d-aspartate (NMDA) receptors prevents the increase in spine density as females transition from diestrus 2 to proestrus, when estrogen levels are rising. Antagonism of NMDA receptors during exposure to the stressful event also prevented the changes in spine density in males and females, despite differences in the direction of these effects. Thus, the stress-induced increase in spine density was prevented in the male hippocampus as was the stress-induced decrease in spine density in the female hippocampus. NMDA receptor antagonism during exposure to the stressful event did not alter corticosterone levels or the corticosterone response to stress. These data suggest that both increases and decreases in spine density can be dependent on NMDA receptor activation.