Previously, we reported that rabbit cerebral arteries contain mast cells that frequently establish close contacts with parasympathetic-like nerve fibers. Here we have examined the possible function of this link by comparing the effects of carbachol and compound 48/80 on mast cell morphology and on the serotonin (5-HT) and histamine content of these arteries. In vivo, 2 micrograms/min of compound 48/80 or 1 micrograms/min of carbachol was infused for 30 min into one internal carotid artery of pentobarbital anesthetized rabbits, the contralateral artery being infused with vehicle. In vitro, the action of 10(-6) M carbachol was tested on isolated middle cerebral artery trees (MCAs) in the presence or absence of 10(-7) M atropine. The effects of carbachol were also tested in vitro on sympathectomized arteries. The 5-HT and histamine contents of all MCAs were measured by radioenzymatic assay, and fragments were prepared for electron microscopy. No histamine was detectable in any artery studied. The 5-HT content of arteries from control animals and those perfused with vehicle (in vivo) or incubated in the physiological solution (in vitro) was 250-300 pmol/mg protein. Both compound 48/80 and carbachol reduced this amount by approximately 50% and induced a marked degranulation of mast cells. Both secretion and degranulation were dramatically blocked in vitro by atropine. No difference in the 5-HT content was observed between intact and sympathectomized arteries under any condition. We conclude that a large proportion of rabbit cerebrovascular 5-HT is stored in mast cells and that cholinergic nerve activation could theoretically release this pool by acting on mast cell muscarinic receptors.