Forty-eight participants completed a working memory span task in a functional magnetic imaging (fMRI) simulator and laboratory. Differences in performance between the two conditions were investigated. The trends in the data indicated that performance in the simulator was impaired in comparison to the laboratory. Significant interactions revealed that performance was impaired for subjects who went into the simulator first to do the span task. Practice effects were degraded in the simulator environment, shown by lower accuracy and working memory capacity scores the second time the task was done. The results suggested impairment in the fMRI simulator was caused by the combined effects of performing a novel task and resisting interference (perhaps to executive attention) from the environmental stressors within the fMRI simulator. Importantly, significant interactions revealed that low span subject’s performance was impaired if they went in the simulator first to perform the span task. These subjects performance was markedly degraded compared to high span subjects by the combined effects of going in the distracting simulator to perform an unfamiliar span task. This could be due to their weaker executive attention component in working memory making them less able to block interference. The study highlights the importance of accustoming and acclimatising subjects, to the experimental task and fMRI environment respectively. Questions are raised about the validity of fMRI studies that do not control for the effects of the fMRI environmental stressors, especially on subjects that are unfamiliar with the experimental task and those that are less adept at performing the task, such as low span subjects.