Carbon monoxide (CO) is a cerebral vasodilator, yet effects of low-level CO exposure (from e.g. smoking) on BOLD FMRI remain unknown. We scanned 12 never-smokers at 3T before and after inhaling low-level CO (or air as a control). CO significantly reduced BOLD response to carbon dioxide during breath holds and attenuated visual cortex activation during visual stimulation and fingertapping, but also increased premotor cortex activation during fingertapping. This indicates that CO generally dampens BOLD signal (possibly through elevated baseline CBF), but that the effect may be task- and/or region-dependent. Caution should be exercised when comparing populations with different CO levels.