Superior sagittal sinus blood flow (Q(ss)) was studied over a 6-h period in nine chronically catheterized fetal sheep as a continuous measure of cerebral blood flow to determine the change in blood flow values and in measures of blood flow variability in relation to behavioural state activity. Mean Q(ss) was increased during the low voltage (LV)/rapid eye movement (REM) state compared to the high voltage (HV)/NREM state by approximately 25%, and was further increased during periods of LV/REM with fetal breathing movements. The increase in Q(ss) was abrupt and began at the transition to LV/REM, with the rate of change 2-fold greater than that during transition to HV/NREM, where the decrease in Q(ss) was gradual and began prior to the evident state change. Q(ss) showed considerable fluctuation, which tended to be greater during the HV/NREM state compared to the LV/REM state when analyzed using measures of longer term variability. Q(ss) thus provides for a continuous measure of cerebral blood flow in the ovine fetus, with the approximately 25% increase with change from the HV/NREM to LV/REM state similar to that previously reported using radioactive microspheres. The abrupt increase in Q(ss) at the transition to LV/REM versus the gradual decrease in Q(ss) before transition to HV/NREM would suggest that the state-related change in brain blood flow is better linked to the presence of the LV electrocorticogram and favours its active generation.