Neurophysiological studies (EEG, ERG, VEP) have been carried out on 8 children with proven GM1 gangliosidosis (3 of Type I and 5 of Type II). All the EEGs were abnormal showing an increasing amount of irregular slow activity as the disease progressed. Around 2 to 3 years of age, Type II patients often showed a fluctuating 4-5 c/s rhythmic activity especially prominent in the temporal regions. Paroxysmal activity was not a conspicuous feature in any of the patients. The ERG was normal in all cases but the VEP was variably altered. The EEG/ERG/VEP findings in GM1 gangliosidosis differ from those seen in most other neurometabolic disorders of childhood.