The number of dendritic spine synapses in the hippocampal CA1 stratum radiatum fluctuates across the rat estrous cycle, being high on proestrus and low on estrus [20]. We hypothesized that the volume occupied by astrocytic processes changes in a complementary manner. The volume fraction of astrocytic processes was determined stereologically in CA1 s. radiatum and s. lacunosum-moleculare of cycling female rats. Consistent with our hypothesis, the volume fraction was significantly lower on the afternoon of proestrus than on the afternoon of estrus in both laminae.