The present study searched for morphological correlates of the permeability of the ventromedial arcuate nucleus of the mouse to blood-borne proteins. First, we determined that highly permeable microvessels are detected in the ventromedial arcuate nucleus using a rat monoclonal antibody to a mouse-specific endothelial phenotype (clone MECA32) recently recognized as a marker of endothelial fenestral diaphragms and previously shown to label circumventricular organs. Second, in the mild conditions of tissue fixation mandatory for use of MECA32, we observed that after a rapid vascular flush with saline, endogenous immunoglobulins are especially retained in circumventricular organs and ventromedial arcuate nucleus. The ventromedial arcuate nucleus thus shares features in common with classical circumventricular organs.