Most circumventricular organs (CVOs) have unusually dense and permeable capillary networks that facilitate secretion of or tissue penetration by circulating substances, unlike other nervous system structures wherein blood-brain barrier (BBB) properties of the capillary endothelium limit solute permeability. In this brief review, I shall discuss new facts from recent experiments, and draw on interpretations from previous studies, to illustrate how capillary systems vary both between and within some CVOs, how closely microvascular properties coincide topographically with the distribution and density of neuropil, transmitter substances and hormonal binding sites, and how physiological data can be combined with morphological descriptions of capillary beds to accent specialized processes of blood-brain solute exchange in individual CVOs. The emphasis of this paper is on exchange microvessels of the rat area postrema (AP), subfornical organ (SFO) and median eminence (ME) which are regions of dense binding for several hormones and contain appreciable numbers of neurons (AP and SFO) or neural terminations that may be part of the sensing apparatus for humoral messengers of homeostatic systems. The work is intended to highlight established concepts about the process of blood monitoring by CVOs, summarize new morphological and physiological characteristics of their capillaries, and provide clues to novel research that could foster further understanding of these curious sentinel and secretory organs of the brain.