['A sudden decrease in arterial blood pressure (ABP) will cause the intracranial blood volume (IBV) to rise, despite the fact that arterial cerebral blood flow decreases. The aim of this study was to test the hypothesis that the increase in IBV is caused by a relative decrease of intracranial venous outflow.', 'In 10 healthy volunteers we studied cerebral autoregulation (CA) by causing an ABP drop with bilaterally deflating leg cuffs. Blood flow velocities (BFV) in the middle cerebral artery and the straight sinus were monitored continuously with transcranial Doppler ultrasound, and the ABP with a non-invasive photoplethysmographic method. After transforming all variables in relative changes, the arterio-venous BFV difference was calculated. Allowing for diameter changes of the intracranial vessels of up to 10 %, and assuming a resting averaged cerebral blood flow of 55 to 60 ml per 100 g brain tissue per minute, an IBV increase of 9 to 10 ml could be calculated.'] Is a sudden arterial blood pressure decrease compensated by an increase in intracranial blood volume?