It has been recently reported that the female steroid hormone 17beta-estradiol enhances synaptic transmission and the magnitude of long-term potentiation (LTP) in adult rodent hippocampus. Moreover, 17beta-estradiol ameliorates cognitive and memory function in postmenopausal women. Since aging is associated with an alteration of synaptic plasticity (e.g., higher susceptibility to long-term depression [LTD]), we examined whether 17beta-estradiol alters the expression of LTD in aged rats. We now report that the induction of LTD recorded from CA1 hippocampal neurons of aged rats is suppressed by 17beta-estradiol treatment, which produced only a minimal effect in suppressing LTD in adult rats. These results suggest that estrogen may act to improve memory by suppressing forgetfulness via a synaptic mechanism, such as LTD.