In order to identify the fundamental features of rat''s eeg, the electrical activities of the neo-, paleo-, andarchi-cortices were simultaneously recorded inunanesthetized curarized albino rats and the following results were obtained. The eeg patterns of the neo-, paleo-, and archicortices in rats were principally the same as those in cats or rabbits. The activation pattern previously reported in cats also seen in rats. Dissociation of the patterns of electrical activity between the neocortex and hippocampus was also seen. Comparison of the different effects of electrical stimulation of the midbrain reticular formation and hypothalamus revealed that the paleo-, and archicortical electrical activity had close functional relation with the hypothalamus. In the limbic cortex, regular slow waves similar to those seen in the hippocampus were sometimes observed.