We examined whether superficial cortical blood flow (CBF) of nonanesthetized rabbits (n = 11) is affected by gravitational (G) stress. Blood flow was measured by laser-Doppler flowmetry through a chronic Plexiglas window implanted over the parietal cortex. Mean arterial blood pressure (MABP) was recorded via an aortic catheter with the pressure transducer situated at the level of the head. The electrocorticogram (ECoG) was also studied in seven rabbits. The animals were exposed to tail-to-head acceleration [6, 10, 15, and 20 G acceleration (Gz) for 1 min]. MABP fell with increasing acceleration (regression line equation: MABP = -7.0 Gz + 129.1; P < 0.001, r = 0.85), whereas CBF was statistically either not different from or greater than control. In one rabbit exposed to 20 Gz, high voltage and slow waves were observed on the ECoG and these changes were preceded by a decrease in CBF to 66% of control. We conclude that the stress and the siphon effect may account for the maintenance of elevated CBF despite low or negative MABP until the occurrence of vascular collapse that induces loss of consciousness.