Hypoctlpnla due to Ilypervontllatlon reduces cerebral blood flow and volumo. To lnvestigate the effects of hyperventilation on the regional signal response to visual activation uslng blood oxygenation level -dependent (BOlO) magnelle resonanee illlaging (MRI), six voJunteors were investigated during vlsual stimulation under norlllocapnia and hypocapnla eondltions. Hyporvenmalion significantly decreased in visual cortex tho BOlD MRI response to vlsual stimulation (3.97 ± 0.5% [mean ( SO) in norlllocapnla vs . 0.77 ± 0.7% in hypocapnia, P < 0.01]. In thrco of six subJects, functlonal signal ehanges were reduced 10 nolse level. Tho redueed stimulus response during hyperventilalion 15 probably duc to a decroasod ovorstlOol in Ihe blood oxygenation response. These results Indlcat~hal BOlO•contrast func-Uonal MRI is highly sensitive to pCO changos . Magn Reson Mcd 41 :213-216, 1999. 0) 1999 Wiley-Liss, lnc.