- Sequential changes in cerebral blood flow and regional distribution were studied in nine chronically catheterized fetal sheep during the transition to the low-voltage ECOG (REM) state to determine the time course for blood flow change within the brain and whether executive centres for REM state generation might thus be identified. Blood flows were measured during the first, second and third minutes after the transition to the low-voltage ECOG state and during the third minute of the subsequent high-voltage ECOG (NREM) state using the radioactive labelled microsphere technique. Blood flow to the brain was increased during the low-voltage REM state when compared to that of the high-voltage NREM state, with the increase evident when measured during the first minute after the state transition and with no sequential change thereafter. Regional blood flow increases during the low-voltage state were greatest to those areas variously associated with the generation of REM state activity, but were again remarkably stable through the first 3 minutes after the state transition. This rapid increase in blood flow within the brain and the regional hierarchy for such, supports the participation of multiple anatomical areas which are highly integrated and act in concert to give rise to what is known as the REM state.