PROJECT SUMMARY In most mammalian species, social interactions among individuals of the same species are governed by dominance relationships. These hierarchical relationships are established and maintained by agonistic behaviors, including aggression. Importantly, recent data indicate that the neural mechanisms underlying aggression and attaining dominance produce a phenotype that is resistant to social stress while the mechanisms underlying subordinate status produce a social stress-susceptible phenotype that may result in a number of adverse behavioral and physiological outcomes. Despite the relationship between social status and stress, the neurochemical mechanisms that underlie dominance have received only limited attention in males and almost no attention in females. This project will fill this critical gap in our knowledge by testing an integrated series of hypotheses using Syrian hamsters and rhesus monkeys. This project will critically test the overarching hypothesis that the agonistic behaviors responsible for the formation and maintenance of dominance relationships are regulated in dramatically different ways by vasopressin (AVP) and serotonin (5- HT) in males and females. Specifically, we propose that activation of AVP and inhibition of 5-HT promotes dominant status and a stress resistant phenotype in MALES while producing subordinate status and a stress susceptible phenotype in FEMALES. In contrast, inhibition of AVP and activation of 5-HT promotes dominance and a stress resistant phenotype in FEMALES while producing subordinate status and a stress susceptible phenotype in MALES. Together, these data will significantly expand our knowledge of sex differences in the neurochemical mechanisms that define social phenotypes and will provide innovative gender specific strategies for promoting resistance to social stress. The data obtained in this project could have an almost immediate clinical impact by guiding drug treatments for stress reduction in men and women as well as guiding drug development by emphasizing the role of AVP-targeted drugs in males and 5-HT- targeted drugs in females.