We studied the effects of parasympathetic nerves on cerebral blood flow (CBF). The greater superficial petrosal nerve, which apparently supplies cholinergic fibers to cerebral vessels and the lacrimal gland, was sectioned on one side at the internal auditory meatus in anesthetized cats. CBF was measured with 15-/un microspheres. Section of the petrosal nerve did not alter resting CBF. In addition, electrical stimulation of the distal cut end of the petrosal nerve had no effect on total CBF. In one area of the brain, the caudate nucleus, stimulation increased blood flow from 29 ± 2 to 38 ± 2 (mean ± SE) ml/min per 100 g. Lacrimal gland blood flow increased from 42 ± 7 to 198 ± 32 ml/min per 100 g during petrosal stimulation, which indicates that the stimulus was potent. In the same experiments, CBF increased 3- to 4-fold during hypercapnia; thus, cerebral vessels were responsive to another dilator stimulus. In other experiments, petrosal nerve section did not alter the response of cerebral vessels to hypercapnia (Pco2> 50 mm Hg) or hypoxia (Po2< 34 mm Hg). We conclude: (1) there is little or no resting vasodilator tone provided to cerebral vessels by the petrosal nerve; (2) petrosal nerve stimulation has a major effect on blood flow to the lacrimal gland but does not increase CBF; and (3) petrosal nerve section has little effect on the response of cerebral vessels to hypercapnia or hypoxia. Circ Res 48: 62-69, 1981