In 21 consecutive cases with unilateral optic neuritis due to demyelinating disease, ERG, C wave of ERG and VER were recorded and studied in both eyes. Also 8 cases suffering from optic atrophy due to trauma or glaucoma were included. The analysis of data shows that the C-wave is significantly diminished in both eyes, namely the affected and the sound, in the cases of optic neuritis. Also in comparison to VER, C wave has a comparative probability in diagnosing optic neuritis due to demyelinating disease. The C wave of ERG is normal in the cases of optic atrophy. The mechanism of C wave involvement during the course of demyelinating disease may be related with an abnormality of the ionic homeostasis of the subretinal space.