Neurons in the ventrolateral division of the hypothalamic ventromedial nucleus (VMNvl) become hypertrophied when exposed to high estrogen levels, an effect that has been observed after estrogen treatment of ovariectomized rats as well as during the proestrus stage of the ovarian cycle. In an attempt to examine whether the neuronal hypertrophy noticed in these conditions reflects metabolic activation of the neurons we have examined, using quantitative methods, the cytoplasmic organelles involved in protein synthesis and the nuclear pores of VMNvl neurons from females on proestrus, when estrogen levels are high, and on diestrus, when estrogen levels are low. Because VMNvl neurons are sexually dimorphic with respect to their size we have performed, in parallel, similar analyses in neurons from age-matched male rats. Our results show that the volume and the surface area of the rough endoplasmic reticulum (RER) and Golgi apparatus are increased at proestrus. They also show that the density of nuclear pores is greater in males than in females whereas the volume and the surface area of the RER and Golgi apparatus are sexually dimorphic only at specific phases of the ovarian cycle: the male-female differences are notorious in the RER when females are on diestrus and in the Golgi apparatus when they are on proestrus. Given that the size of the RER and of the Golgi apparatus correlates with the level of neuronal protein synthesis, data obtained in this study suggest that the sex-related differences and the estrus cycle variations in neuronal size reflect corresponding differences and fluctuations in the metabolic activity of VMNvl neurons.