Spindle-like afterdischarges evoked by stimulation of ventrolateral thalamic nucleus and spontaneous spindle waves were analized to investigate the electrophysiological mechanism of cortical synchronized activities in man as well as in cat. In man, three positive and three negative waves, namely Ⅰ-P, Ⅰ-N, Ⅱ-P, Ⅱ-N, Ⅲ-P and Ⅲ-N, were observed as cortical evoked responses after single stimulation of the ventrolateral thalamic nucleus. And Ⅳ-P and Ⅳ-N were observed after the stronger stimulation which could provoke augmenting response with the repetitive volley. Later components of evoked cortical responses were investigated in cat immobilized under local anesthasia. Recording electrole was located on the anterior sigmoid gyrus. As the intensity of the thalamic stimulation was increased, afterdischarges of the negative waves, which appeared with long latency (about 200 msec.), increased in number and developed into spindle-like pattern with notches on afterdischarges. The long latencied negative wave in cat and Ⅳ-N in man were considered to correspond to Lehtinen's "precursor of the evoked spindle activity". It has been already reported by Miyamoto that Ⅳ-N was supposed to play an important role in augmenting response in man. Close relationship was indicated between augmenting response and spindle-like afterdischarges. Futhermore, the similarities of the pattern between spindle-like afterdischarges following thalamic stimulation and spontaneous spindle waves in cat, and attenuation of ipsilateral spontaneous spindle waves after destruction of the ventrolateral thalamic nucleus in cat as well as in man were also observed. From these facts these cortical synchronized activities, namely AR, spindle-like afterdischarges and spontaneous spindle waves were considered to be brought by similar neuronal mechanism with some varieties of synchronization ascribed to arousal levels under influences of activities of the ventrolateral thalamic nucleus. On the other hand, spindle-like afterdischarges and spontaneous spindle waves in cat were analized for investigating their waxing and waning phenomena. Notches were observed on each waves. When the wave grew with maximal amplilude in a train of spindle, the notch of the wave became obscure. The phenomenon was considered that the notch was synchronized with the wave, forming maximal amplilude of the wave in a train of spindle. In other words, mechanism of this phenomenon was supposed to be explained as a beat of two rhythmic activities slightly different in frequency.