Body fluid homeostasis is maintained by integrated hormonal and behavioral mechanisms, including water intake. The majority of research on this topic has been done in male animal models. However, it is becoming increasingly clear that females respond differently to dehydration than do males. In fact, there is general agreement that estrogen modulates water intake in response to dehydration. Despite this general agreement, there are conflicting findings about the role of estrogen in the responses to specific dehydrational challenges. In addition, methodological differences in hormonal manipulations make these studies difficult to assess. More importantly, very little is known about the neural mechanisms and central pathways underlying the effect of estrogen on water intake. Most studies that have examined this issue have used central administration of angiotensin II to stimulate water intake. Although results from these studies provide compelling evidence for a central estrogen/angiotensin II interaction in the control of water intake, it is unclear whether such an interaction is involved in more physiological stimuli for water intake. Accordingly, the goals of the present proposal are to distinguish estrogen effects on drinking behavior and ascertain the mechanism by which the hormone affects the thirst of female rodents. By doing so, we will test the overall hypothesis that central interactions between estrogen and angiotensin II underlie the effect of estrogen on physiologically stimulated water intake.