In this review, the emerging functional roles of the brain angiotensin system have been considered. The major effects of Ang II can be classified into three groups, which imply three possible functions: The first, and largest, group is actions associated with the regulation of body fluid volume in response to hypovolemia. These include thirst, blood pressure increase, vasopressin release, sodium appetite and excretion, and ACTH and aldosterone release. This function alone has important implications for the control of blood pressure and the disease of hypertension. Another possible function is a role for angiotensin in the activity of gonadotropic hormone releasing hormones and pituitary hormones during the reproductive cycle and pregnancy. A third group of functions is the synaptic, neurotransmitter interactions of Ang II with catecholamines, serotonin, prostaglandins, and other peptides, not all of which could be reviewed here due to space limitations. This interaction is significant for all functions mentioned and leads to alterations in motivation (thirst, pain), memory (and possibly learning), and motor control. The amount of data available, however, is so limited that to claim angiotensin plays any major role in the latter functions would be premature. Throughout this review, we compared the central and peripheral effects of Ang II. We suggest that normally, a blood-CVO barrier prevents diffusion of peripheral Ang II to brain receptors inside the BBB. Because of this mechanism, the responses to the two routes of administration are distinctly different. When systemic peptide levels are low, Ang II activates only receptors in the CVOs; however, when these levels are high, the peptide diffuses to receptors that are normally activated only by brain Ang II.