The effects of electrical stimulation of the pressor area of the fastigial nucleus (FN) of cerebellum on regional cerebral blood flow (CBF) were examined in anesthetized, paralyzed rats. CBF was measured in tissue homogenates by the Kety principle using 14C-iodoantipyrine as an indicator during FN stimulation, during stimulation of the vermal cortex, or after placement of electrodes in FN (unstimulated controls). Blood gases and systemic arterial pressures were comparable in the three groups. FN stimulation elicited a widespread and significant (P less than 0.005) increase in CBF in 12 of 13 areas. Flow was maximally increased in cerebral cortex (up to 220% of control) but also rose in selected regions of telencephalon, diencephalon, mesencephalon, lower brain stem, and in white matter of the corpus callosum (172%). Flow was not increased in cerebellum. The increase in CBF persisted after transection of the spinal cord at C1 or cervical sympathectomy. In five rats, hypercarbia (PCO2 to 63 mmHg) increased CBF in all 13 regions but in a pattern differing from FN stimulation. Excitation of neurons originating in, projecting to, or passing through FN can elicit a potent and virtually global increase of CBF. The effect appears mediated by intrinsic pathways of the central nervous system.