Individuals are routinely exposed to low-level carbon monoxide (CO), by factors such as ambient pollution and tobacco smoking. It is known that inhalation of high levels of CO have a detrimental impact on cognitive function. This study sought to investigate the impact of low-level CO exposure on central nervous system cognitive processing speed, using Blood Oxygen Level Dependant (BOLD) functional Magnetic Resonance Imaging (fMRI). The effects of low-level CO (raised up to 6ppm in exhaled air) on reaction times and fMRI activation maps were measured in healthy non-smoking participants. Participants received BOLD fMRI scans on two separate occasions (air and CO intervention days) and were scanned during the performance of a simple reaction time task. Results showed mean activation in cerebellum and motor cortex for all conditions. A significant reduction in BOLD response in the right temporal gyrus was found following CO inhalation, compared to the air control. Reaction times were significantly slower after CO exposure on the CO experimental day, but did not significantly change on the air control experimental day. This suggest that even low-level CO may impact both behavioural and BOLD fMRI outcomes.