Recently, the origins and pathways of cerebrovascular acetylcholine- and vasoactive intestinal polypeptide-containing nerves have been elucidated in detail in the rat: The sphenopalatine ganglion is the major source for postganglionic parasympathetic fibers to the vascular beds of the cerebral hemispheres. To clarify the functional role of the nerves on cerebral blood vessels in vivo, brain cortical microvascular blood flow was measured in rats during electrical stimulation of these particular postganglionic fibers. Animals were subjected to transection of the right nasociliary nerve 2 weeks before the flow measurements to eliminate activation of peptidergic sensory fibers. Relative change in microvascular blood flow was continuously recorded by a laser-Doppler flowmeter system under alpha-chloralose anesthesia. The postganglionic fibers were electrically stimulated just proximal to the ethmoidal foramen by a bipolar platinum electrode (5 V; 0.5 ms; 3, 10, 30, 60 Hz; as a continuous stimulation for 90 s). Stimulation at 10 Hz induced a marked increase of the cortical blood flow (CoBF) on the ipsilateral side, whereas no change was observed on the contralateral side. It reached a maximum mean value of 42.5% at 46 s, and then slightly declined during the remaining stimulation period. No significant changes were observed in the mean arterial blood pressure or blood gases during or after stimulation. Both atropine and scopolamine failed to alter this flow increase. Electrical stimulation of the postganglionic fibers at different frequencies revealed a maximal increase in the CoBF at 30 Hz in the control situation (47.2%), but at 10 Hz after scopolamine administration (51.6%).(ABSTRACT TRUNCATED AT 250 WORDS)