Using the single-section Golgi impregnation technique, sex differences in hypothalamic ventromedial (VMN) neurons of gonadectomized juvenile and peripubertal rats were assessed. The effect of estrogen treatment on VMN neurons was also investigated. Juvenile rats were gonadectomized at 16 days of age and peripubertal rats at 36 days. At 5 days after surgery, the rats were injected with estradiol benzoate (20 micrograms/kg) or sesame oil for 2 days after which they were perfused and the brains processed for Golgi impregnation. Estradiol benzoate treatment significantly increased dendritic and soma spine density in juvenile and peripubertal male and female rats. A sex difference was observed in oil- and in estradiol-treated rats, with females exhibiting higher dendritic spine density and a greater response to estrogen priming. Moreover, dendritic and soma spine density was significantly higher in juvenile versus peripubertal rats. It is possible that the sex difference observed in dendritic and soma spine density and in the response to estradiol benzoate treatment is due to an organizational effect of sex steroids.