Blood volume, blood flow, and blood-to-tissue transfer of an amino acid in circumventricular organs, such as the median eminence and subfornical organ, and the pituitary gland of conscious rats were measured by using quantitative autoradiographic techniques and computer-assisted processing of the tissue images. Retained erythrocyte and plasma volumes observed in circumventricular organs and the anterior and neural lobes of the pituitary gland were dissimilar but in all cases greater by several times than those in cerebral grey matter; these findings suggest the presence of a dense network of high-resistance microvessels in circumventricular organs. The rate of capillary blood flow in the subfornical organ and median eminence was similar to that of grey matter, whereas blood flow in the pituitary neural lobe was several times higher than in grey matter. Thus the apparent velocity of intracapillary blood flow is much higher in the neural lobe than in the subfornical organ. Blood-to-tissue transfer of a small neutral amino acid, alpha-aminoisobutyric acid, was 200 to 700 times more rapid in circumventricular organs and pituitary neural lobe than in the inferior colliculus and caudate nucleus, structures having a blood-brain barrier (BBB). Morphometric analyses indicated that capillary volume and surface area were two times larger in the neural lobe than in the subfornical organ. Moreover, capillaries of the neural lobe and subfornical organ had numerous endothelial fenestrations and cytoplasmic pits or vesicles, whereas capillaries of the inferior colliculus had no fenestrations and fewer vesicles. These studies demonstrate quantitative differences in the microcirculatory systems not only between circumventricular organs and BBB structures but also among circumventricular organs.