The "periodic complex" associated with subacute sclerosing leucoencephalitis is highly responsive to alterations in body temperature. At an elevation in body temperature, the complexes progressively decrease in frequency and amplitude and are eventually abolished. A lowering of the body temperature has the opposite effect. Studies of the VER and H reflex at varying temperatures suggest that the changes in the periodic complexes are mediated by alterations in synaptic activity. © 1971.