We sought to determine whether the increase in regional cerebral blood flow (rCBF) elicited by electrical stimulation of the fastigial nucleus of the cerebellum (FN) is secondary to, or independent of, increased local cerebral metabolism (rGMR) in anesthetized (chloralose) paralyzed rats. rCBF and rGMR were determined autoradiographically in separate groups of animals with comparable blood gases and systemic pressure, by the [14C]iodoantipyrine and [14C]2-deoxyglucose methods respectively. In sham-operated controls, rCBF (n = 5) and rGMR (n = 5) were closely related in the 28 brain areas studied (r = 0.733; P less than 0.005). During FN stimulation, rCBF (n = 6) increased significantly in 24 of the 28 areas, the greatest increase being in the cerebral cortex (up to 215%). rGMR (n = 9) increased in only 15 areas, so that the correlation between rCBF and rGMR throughout the brain became weaker (r = 0.568; P less than 0.005). Where the rCBF increases were the greatest (particularly in the cerebral cortex), rGMR was unchanged. This suggests that the rCBF increase was not a consequence of the increased rGMR. We conclude that neurons originating in or passing through FN may influence local cerebral circulation through a primary cerebral vasodilatation not coupled to metabolism.