Circumventricular organs (CVOs), small structures bordering the ventricular spaces in the midline of the brain, have common morphological and endocrine-like characteristics that distinguish them from the rest of the nervous system. Among their unique features are cellular contacts with two fluid phases--blood and cerebrospinal fluid--and neural connections with strategic nuclei establishing circuitry for communications throughout the neuraxis. A variety of additional morphological and functional characteristics of the CVOs implicates this group of structures in a wide array of homeostatic processes. For three of the circumventricular organs--the subfornical organ (SFO), the organum vasculosum of the lamina terminalis (OVLT), and the area postrema (AP)--recent findings demonstrate these structures as targets for blood-borne information reaching the brain. We propose that these three sensory CVOs interact with other nuclei in the maintenance of several homeostatic processes by way of neural and humoral links. We emphasize the collective role of brain CVOs in the maintenance of body fluid homeostasis as a model for the functional integration of these fascinating "windows of the brain" within central neurohumoral systems.