The changes produced in the hypothalamic ventromedial nucleus (VMN) of ovariectomized rats after administration of 100 microgram estradiol benzoate/kg body weight were studied using light and electron microscopy. Quantitative morphometric studies included number and size of VMN neurons and nuclei, size and density of terminals and synaptic contacts, spine-to-shaft ratio of postsynaptic elements and relative frequency of two types of synaptic vesicles. Evidence was obtained favoring the concept of heterogeneous composition of the VMN: in ovariectomized animals many cells appeared in a state of quiescence, but other neurons showed no major alterations. Estrogen administration to ovariectomized rats produced evidence of metabolic stimulation such as increase in rough surfaced endoplasmic reticulum, condensation of nucleolar material, enlarged Golgi and presence of pleomorphic mitochondria. The number of neurons in the VMN was not modified by estrogen treatment; however, neuron soma and nuclei were larger. In the ventrolateral division of the VMN terminals and synaptic contacts per unit area were increased after estrogen treatment, but synaptic contact length, terminal size and spine-to-shaft ratio were not modified. The possibility that the differences observed may be consequent to changes in synaptic organization of the VMN related to its estrogen-dependent functions is discussed.