Eighteen multiple sclerosis (MS) patients with a history of optic neuritis and ten normal subjects had the visual critical flicker fusion frequency (CFF) determined prior to and during exposure to moist hot air. Central temperature was monitored by a thermocouple placed against the tympanic membrane. The patients demonstrated a dramatic fall in CFF on exposure to heat, while normals showed a slight rise in CFF. The effect was frequently exquisite: in one instance a 40% fall in CFF was correlated to a 0.2 C rise in central temperature. The effect was also quite rapid with a statistical difference between the normal and patient responses noted within two minutes after heat exposure. During this time there was little demonstrable change in central temperature. Prediction curves for the CFF change of heated patients and normals were also calculated.