The ventromedial hypothalamus (VMH) is a sexually dimorphic brain region consisting of distinct subdivisions, including the dorsomedial (VMHdm) and ventrolateral (VMHvl) regions. Specifically, the VMHdm is associated with sociosexual behaviors such as scent marking and ultrasonic vocalizations, while the VMHvl is associated with lordosis behavior in females and mounting behavior in males, as well as male-male aggression. These sexually differentiated adult behaviors mature during puberty, although the mechanisms that underlie these emergent sex differences remain unknown. Previous studies in rats and hamsters show that pubertal hormones promote the addition of new cells to sexually dimorphic brain regions [1]. Furthermore, some of these pubertally born cells differentiate into mature neurons or glia and are functionally incorporated into adult neural circuits [2]. As the mouse model offers a diverse selection of genetic tools to investigate hormonal influences on pubertal cytogenesis, the current study examined whether new cells are added during puberty to the mouse VMH, and if so, whether there are sex differences. During puberty, from postnatal day (PND) 28-49, male and female mice were given daily injections of bromodeoxyuridine (BrdU), a cell birthdate marker. All animals were sacrificed on PND 60, when the age of the BrdU-labeled cells ranged from 11-32 days. Immunohistochemistry revealed BrdU-labeled cells in the VMHdm and VMHvl. Quantitative analysis using Neurolucida software revealed a similar density of BrdU-labeled cells (# BrdU-labeled cells/mm3) in the VMHdm of males and females (F=0.001, p=0.98); however, in the VMHvl, males had ~1.3 times more BrdU-labeled cells than females (F=10.11, p<0.03). Thus, pubertal cytogenesis occurs in the mouse brain and more pubertally born cells survive into adulthood in the VMHvl of males than females, suggesting that these cells may play a role in the emergence of sex-specific adult behaviors, such as male-male aggression [3].