We used steady-state susceptibility contrast MRI to evaluate the regional cerebral blood volume (rCBV) response to hypocapnia in anesthetised rats. The rCBV was determined in the dorsoparietal neocortex, the corpus striatum, the cerebellum, as well as blood volume in extracerebral tissue (group 1). In addition, we used laser-Doppler flow (LDF) measurements in the left dorsoparietal neocortex (group 2), to correlate changes in CBV and in cerebral blood flow. Baseline values, expressed as a percentage of blood volume in each voxel, were higher in the brain regions than in extracerebral tissue. Hypocapnia (P a CO 2 % 25 mmHg) resulted in a significant decrease in CBV in the cerebellum (À17 AE 9%), in the corpus striatum (À15 AE 6%) and in the neocortex (À12 AE 7%), compared to the normocapnic CBV values (group 1). These changes were in good agreement with the values obtained using alternative techniques. No significant changes in blood volume were found in extracerebral tissue. The CBV changes were reversed during the recovery period. In the left dorsoparietal neocortex, the reduction in LDF (group 2) induced by hypocapnia (À21 AE 8%) was in accordance with the values predicted by the Poiseuille's law. We conclude that rCBV changes during CO 2 manipulation can be accurately measured by susceptibility contrast MRI.