The blood-brain barrier consists of endothelial cells, pericytes, and astrocytic endfeet and protects neurons from circulating hazardous molecules. The fenestrated microvessels of the sensory circumventricular organs (CVOs), including the organum vasculosum of the lamina terminalis, subfornical organ, and area postrema, shows a less restrictive barrier than is found in other areas of the brain to sense a variety of plasma molecules and convey their information into other brain regions. In the present study, we aimed to examine how parenchyma entry of circulating molecules is restricted in the sensory CVOs. The previous and present tracer assays revealed that the molecules more than or equal to molecular mass (MM) 10,000 stayed in the perivascular space between the endothelial and parenchymal basement membranes. On the other hand, low-molecular-mass tracers such as FITC (MM: 389) entered into parenchyma but did not pass beyond the area of dense astrocytic network. The immunoreactivity of tight junction proteins of claudin-1, occludin and zonula occludens-1 was seen between astrocytic cellular processes in the sensory CVOs, suggesting that such intercellular bindings may have barrier functions, especially for low-molecular-mass factors. By using a selective astroglial toxin L-α-amino-adipic acid, we demonstrated that the fragmentation of astrocytic processes resulted in increased low-molecular-mass tracer entry into parenchyma. The present study demonstrates that high-molecular-mass tracer is trapped at perivascular space and entry of low-molecular-mass tracer into parenchyma is limited by astrocytic dense network, indicating that there are alternative barrier that protect neurons from toxic molecules and make it possible to sense circulating signals in the sensory CVOs.