Reactions of pial arteries to the muscarinic agonist carbachol were tested in vivo in chloralose anesthetized cats before and after endothelial damage. Moderate endothelial damage was induced by arterial air embolism and verified by electron microscopy for the vessels tested. The experiments had three phases; First, the normal reactivity of pial arteries to carbachol (10(-7) to 10(-5) M) was tested using the microapplication technique, then, after air embolism, the reactivity was reinvestigated at the same vessel. Finally, pial arteries were taken out for scanning electron microscopy. The results show carbachol (10(-6) and 10(-5) M) induced significant dilations under control conditions, also after repetition at the same vessel. After air embolism, the reactions to carbachol were abolished. Morphologic data revealed that whereas control pial arteries showed intact endothelium, the embolized vessels revealed various degrees of endothelial alterations. All showed flattening of endothelial nuclei, to a greater or lesser degree, and in many cases, the endothelium had a wrinkled appearance; several arteries showed severe degradation of the intercellular junctions. It is concluded that (a) carbachol-induced muscarinic vasodilatation of pial arteries in vivo can be abolished after a morphologically verified endothelial lesion--thus confirming in vitro studies in larger arteries and (b) disturbed vascular function does not require rubbing of the endothelium, but occurs already with moderate endothelial damage.