The ultrastructure and permeability to the marker horseradish peroxidase in the leptomeningeal and neural vessels of the optic lobes, and in the wing bud vessels, were compared, during early stages of chick embryo vasculogenesis, in order to ascertain whether young neural vessels, like mature ones, possess structural specificity preventing the free transport of molecules through their wall (blood-brain barrier). The results demonstrated that immature endothelia are always permeable to the tracer, since the marker passes the vessel wall through the interendothelial clefts (paracellular route) as well as by endo-exocytotic mechanisms (transcellular route). Nevertheless, the transport by vesicles is somewhat reduced in the neural vessels compared to that in the extraneural ones, suggesting that the former may be influenced by the surrounding neuropile to differentiate in a specific manner right from early development.