This study describes the localization of endothelin-1-, vasopressin- and substance P-like immunoreactivity in a subpopulation of endothelial cells of the basilar and posterior communicating arteries of the (i) normal rabbit brains and (ii) rabbit brains perfused with a perfluorocarbon emulsion (PFC). Following perfusion with the PFC at increased flow the endothelial cell ultrastructure appeared normal, although there was a decrease of immunoreactivity to these peptides. This suggests that these peptides may be involved in endothelial control of blood flow in cerebral vessels.