A visual psychophysical adaptation procedure was used on patients with Multiple Sclerosis (MS) in an attempt to induce a temporary and local exacerbation of subclinical visual impairment. Using a flicker detection task, sensitivity before and after adaptation to a flickering stimulus was measured in 9 MS patients and 9 control subjects. Although only 22% of patient eyes had abnormal flicker sensitivity prior to adaptation, visual deficit was observed in 83% of eyes studied after adaptation. Of the 7 MS eyes studied for which no other sign or symptom of visual involvement was present, 5 were found to have visual deficits after flicker adaptation. In addition, 10 of the 11 eyes affected by MS showed an abnormal response to flicker adaptation. Recovery from the effects of adaptation was complete in all patients within 2 minutes. The results suggest that partial demyelination of visual pathway neurons may exist in patients without signs or symptoms of visual involvement. The prolonged stimulation provided during adaptation may produce a temporary fatiguing or conduction blockade of such neurons which may lead to reductions in sensory sensitivity.