Our reports on the electrical activity of the brain in epilepsy have thus far been almost entirely descriptive.1Interpretation, we believed, could wait until a considerable body of data had been acquired. Now, after two and one-half years, we have electro-encephalograms for over four hundred patients. We have daily records for some patients extending over many days; others have been studied repeatedly at long intervals, and for others we have made long-continued records, the longest being thirty-four hours. Altogether, we have a total of approximately 60 miles (96 kilometers) of records. Most of these records were made with a multichannel instrument recording simultaneously from four parts of the head. The object of this effort has been to secure electro-encephalograms of patients during seizures, but we also wished to see what abnormalities, if any, occurred in the interval between seizures. We have records of the grand mal seizures of thirty