We have used Golgi-impregnated tissue to demonstrate that apical dendritic spine density in CA1 hippocampal pyramidal cells undergoes a cyclic fluctuation as estradiol and progesterone levels vary across the estrous cycle in the adult female rat. We observed a 30% decrease in apical dendritic spine density over the 24-hr period between the late proestrus and the late estrus phases of the cycle. Spine density then appears to cycle back to proestrus values over a period of several days. In contrast, no significant changes in dendritic spine density across the estrous cycle occur in CA3 pyramidal cells or dentate gyrus granule cells. These results demonstrate rapid and ongoing dendritic plasticity in a specific population of hippocampal neurons in experimentally unmanipulated animals.