Increases in cerebral blood flow produced by vasoactive agents will increase blood oxygen level-dependent (BOLD) MRI signal intensity. The effects of such vasodilation on activation-related signal changes are incompletely characterized. The two signal changes may be simply additive or there may be more a complex interaction. To investigate this, BOLD MRI was performed in four normal male subjects using T2*-weighted echo planar imaging; brain volumes were acquired every 6.2 s, using a Siemens VISION scanner operating at 2 Tesla; each volume consisted of 64 sequential transverse slices (64 × 64 pixels per slice, 3 × 3 × 3 mm). Sixteen periods of visual stimulation were produced using a flickering checkerboard (8 Hz, 31 s On/31 s Off); this was coupled with five periods of hypercapnia (4% inspired CO2, 62 s On/124 s Off). Data were analyzed using SPM96. Mean signal intensity, calculated globally for the whole brain, closely mirrored changes in the partial pressure of end-tidal CO2 (PCO2), and hypercapnia was associated with wide-spread significant signal increases (P <0.001), predominantly within grey matter. As expected, the visual stimulation produced significant signal changes within the occipital cortex (P <0.001). Within the occipital cortex, no significant interactions (P > 0.001) between the effects of the visual stimulation and PCO2 were present. The increases in PCO2 imposed dynamically in the present study would increase cerebral blood flow by between 25 and 40%, an increase within the physiological range and comparable to that induced by neural activation. With this flow change the effects of vasodilation, on an activation-related signal change, are simply additive. © 2001 Academic Press.