Internuclear ophthalmoparesis (INO) in multiple sclerosis (MS) is due to demyelination of the medial longitudinal fasciculus (MLF) and provides an accessible model for studying consequences of raised body temperature and fatigue on central demyelination.1,2 Prompted by one of our patient’s report of vision improvement after initiating dalfampridine, a potassium channel blocker prescribed for gait impairment,3 we measured this drug's effects on 3 patients with MS with bilateral INO. All showed changes in horizontal saccadic conjugacy consistent with improved transmission of the neural pulse responsible for adducting movements.