Low doses of estrogen (E) were given to ovariectomized (OVX) rats and the ventromedial hypothalamic nucleus (VMN) was examined at the ultrastructural level. Administration of 10 micrograms of E for two consecutive days resulted in an increase in the number of axodendritic synapses in the VMN in comparison to OVX rats treated with oil. The percentage of axospine and multiple synapses and the length of the postsynaptic density were unchanged by E treatment. These results provide further support for the ability of low doses of E to alter neuronal morphology.