Oxytocin (Oxt) and vasopressin (Avp) are structurally similar neuromodulators of sex-specific social behavior across mammalian species. Due to their structural similarities, behavioral effects, and presence in the brain from embryogenesis to adulthood, examining how they alter behavioral output can shed light on commonalities between mammalian species and how social behavior is modulated across the lifespan. Previous work in multiple rodent species found that manipulations of the Oxt receptor (Oxtr) just after birth results in sex-dependent changes in neuroanatomy and behavior in adulthood; in some cases altering the Avp system. Mice with lifelong dysfunctions in their Oxt or Avp systems have similar behavioral abnormalities that vary by sex, including deficits in social memory and communication, as well as changes to male aggressive behaviors. Together, these findings suggest the Oxt and Avp systems play an important role in development, perhaps contributing to the sex-specific development of brain circuitry that is important to the neural regulation of social behavior. However, to date, much of this work has focused solely on the Oxt system, and there has been no direct connection between the actions of the developing Oxt system and adult behavior. Moreover, while these neuropeptide systems are present during embryonic development, most studies have focused on the postnatal period, and largely in only one sex - males. Work from our lab has shown that while females produce Oxt mRNA by E14.5, males do not produce Oxt mRNA until after birth despite both sexes producing the Oxtr by E16.5, suggesting the that the Oxtr's sex-dependent actions may begin at this time. This dissertation focuses on the ways in which the embryonic Oxt and Avp systems differ between males and females, and what the consequences are for the expression of social behaviors later in life. In Chapter 2, the timecourse of expression for the embryonic Avp system in males and females was determined. In Chapter 3, a functional link was made between embryonic Oxtr signaling and sex-dependent behavioral expression in adulthood. Finally, in Chapter 4, how gene expression varied between males and females at E16.5, with implications for the developing Oxt and Avp systems, was evaluated. Based on these findings, it was concluded that Oxt and Avp systems likely act to organize the brain differently in males and females, which ultimately results in context-appropriate behavioral expression in adulthood.