Cerebrospinal fluid (CSF) provides hydraulic suspension for the brain. The general concept of bulk CSF production, circulation, and reabsorption is well established, but the mechanisms of momentary CSF volume variation corresponding to vasoreactive changes are far less understood. Nine individuals were studied in a 3T MR scanner with a protocol that included visual stimulation using a 10-Hz reversing checkerboard and administration of a 5% CO2 mix in air. We acquired PRESS-localized spin-echoes (TR = 12 sec, TE = 26 ms to 1.5 sec) from an 8-mL voxel located in the visual cortex. Echo amplitudes were fitted to a two-compartmental model of relaxation to estimate the partial volume of CSF and the T2 relaxation times of the tissues. CSF signal contributed 10.7 ± 3% of the total, with T2,csf = 503.0 ± 64.3 [ms], T2,brain = 61.0 ± 2 [ms]. The relaxation time of tissue increased during physiological stimulation, while the fraction of signal contributed by CSF decreased significantly by 5-6% with visual stimulation (P <0.03) and by 3% under CO2 inhalation (P <0.08). The CSF signal fraction is shown to represent well the volume changes under viable physiological scenarios. In conclusion, CSF plays a significant role in buffering the changes in cerebral blood volume, especially during rapid functional stimuli. © 2009 Wiley-Liss, Inc.