The mammillary bodies of the posterior hypothalamus are one of the CNS structures in which structural sexual dimorphism has already been described. This study quantifies the argyrophilic nucleolar organizer regions (NORs) of the neurons from this region in the male and female rats, the latter during two major phases of the estrous cycle (estrus-diestrus). The number and relative area of these NORs stained with a silver nitrate technique are considered as an index of the global protein synthesis of the neurons in the different nuclei from the mammillary bodies. Our results show the existence of statistically significant differences between sexes and estrous cycle phases. These findings suggest a significant influence of gender and the hormonal state on the neural activity of the MB.