We investigated the effects of cortical stimulation on mental rotation tasks in a patient with an electrode array placed over his left primary motor cortex. The array was implanted to relieve chronic pain resulting from right brachial plexus damage. Tasks involving motor imagery were slowed down by cortical stimulation, whereas those involving visual imagery were not. When the patient performed the motor-imagery task, the interference effect on response times disappeared if the stimulator was switched off. We also probed two of the sites (anterior-lateral and posterior-medial position), and found that stimulation of the more anterior-lateral one consistently disrupted motor imagery.