A pilot study was conducted on two occasional cannabis consumers, in order to investigate the effects of cannabis smoking on tracking capabilities. The subjects participated to a cross-over fMRI experiment based on a visuo-motor tracking task, once before and once after smoking, either cannabis or a placebo. We quantified the performance of the subjects by measuring the precision of the behavioral responses (i.e. percentage of time of correct tracking, reaction times to swerves) during the fMRI session. Interestingly, while the first subject had decreased performances after cannabis inhalation, the second subject performed the task with more accuracy after cannabis smoking. Maximum THC concentrations peaked at respectively 151 and 227 ng/ml whole blood. Blood THC values during fMRI, interpolated from concentrations determined before and after the experiment, were in the range of 12 to 8 ng/ml for the first subject and of 20 and 11 ng/ml for the second subject. The fMRI BOLD response revealed two effects of cannabis inhalation common to both subjects, namely a global decrease of activation of the visuo-motor areas, and the increase of activation of a fronto-parietal attentional network. Moreover, a supplementary activation in prefrontal and orbitofrontal areas was constantly found during tracking in the subject who performed more accurately after smoking; this activation was not present in the other subject with cannabis-related performance impairment. Taken together, these results suggest that the two subjects may follow different patterns of cerebral activation while performing the visuo-motor tracking task under the influence of cannabis. Recruitment of supplemental brain areas might compensate somehow the global decrease of brain activation after cannabis inhalation and partly mitigate harmful effects on tracking performances. These preliminary results need to be confirmed with more subjects.