We describe a set of perivascular interneurons (PINs) originating a series of fibro-vesicular complexes (FVCs) throughout the gray matter of the adult rabbit and rat brain. PINs-FVCs are ubiquitous throughout the brain vasculature as defined in Golgi-impregnated specimens. Most PINs consist of small, aspiny cells with local or long (> 1 millimeter) axons that split running with arterial blood vessels. Upon ramification, axons originate FVCs around the roots of the arising vascular branches. Distally, FVCs form paired axons that run parallel to the vessel's wall until another ramification ensues and a new FVC is formed. This alternating pattern ceases when the capillary diameter narrows (i.e., <8 µm) and axons resolve. FVCs, as visualized by electron microscopy, consist of clusters of anastomotic perivascular bulbs (PVBs) arising from the PIN's unmyelinated axon. PVBs lie alongside the pre-or -capillary wall, surrounded by end-feet and the extracellular matrix of endothelial cells and pericytes. A PVB contains mitochondria, multivesicular bodies, and granules with a membranous core similar to those observed in Meissner corpuscles and other mechanoreceptors. Some PVBs form asymmetrical, axo-spinous synapses with presumptive adjacent neurons. Antisera to sensory fiber-terminals co-label putative FVCs that are embedded by astrocytic end-feet. Because of the strategic location, ubiquity, and cytological organization of the PIN-FVC, it is suggested that: 1. PIN-FVCs are distributed throughout the mammalian brain vasculature. 2. The PIN-FVC is a putative sensory receptor intrinsic of the neurovascular unit. 3. The PIN-FVC may correspond to an afferent limb of the sensory-motor feed-back controlling local blood flow.