Electrophysiological studies on evoked cortical potentials following ventrolateral (VL) thalamic stimulation of man were performed with averaging computer technique for 41 patients of parkinsonism and other involuntary movement disorders during stereotaxic surgery for last 2 years. 1. Single stimulation of the VL nucleus demonstrated bilateral cortical activity with I-PN (first positive negative), Ⅱ-PN (second positive negative), Ⅲ-PN (third positive negative) and Ⅳ-PN (fourth positive negative) waves. Ⅲ-P wave was frequently superimposed on negative response between Ⅱ-N and Ⅲ-N wave, and Ⅳ-PN waves were frequently feeble in single stimulation of the VL nucleus. 2. Peak latencies of these waves were estimated 3.4±1.2 msec in Ⅰ-P, 10.6±1.8 msec in Ⅰ-N, 29±5 msec in Ⅱ-P, 55±6 msec in Ⅱ-N, 71±11 msec in Ⅲ-P, 90±11 msec in Ⅲ-N, 115±19 msec in Ⅳ-P and 160±24 msec in Ⅳ-N wave in ipsilateral central lead, and 4.0±1.5 msec in Ⅰ-P, 11.1±2.1 msec in Ⅰ-N, 31±4 msec in Ⅱ-P, 57±6 msec in Ⅱ-N, 74±13 msec in Ⅲ-P, 92±12 msec in Ⅲ-N, 118±24 msec in Ⅳ-P and 156±29 msec in Ⅳ-N wave in contralateral central lead. 3. The first deflection time of Ⅰ-P wave, which meant beginning of the evoked response, was 1.5-1.8 msec in stimulated side of central cortex and 2.2-2.9 msec in contralateral central lead, which were obtained in 5 cases precisely measured. 4. The impulse, which provoked Ⅰ-P N waves, was thought to be conducted from stimulated VL nucleus to the contralateral cortex directly via corpus callosum with 36-43m/sec of velocity. 5. Cortical evoked responses following suprathreshold low frequency stimulation (5-12Hz) of the VL nucleus showed invariably augmenting response and recruiting-like augmenting response. Augmenting response, which consisted of a train of growth in both. positive and negative components, was evoked when each stimulus was given on the descending phase from the peak of Ⅲ-N or augmented negativiy to the bottom of following positive wave of the preceding response. Analysis of augmentation suggested that synchronization of preceding evoked Ⅳ-P and present Ⅱ-P wave would be occured in augmented positivity and synchronization of preceding evoked Ⅳ-N and present Ⅱ-N or Ⅲ-N wave would be occured in augmented negativity. Recruiting-like augmenting response was obtained when each stimulus was given on the ascending phase from the bottom of deep Ⅳ-P to the following negative wave of the preceding response. Recruiting-like augmentation was shown to be a similar response as augmenting response in the fundamental pattern of averaged evoked activity, although development of negativity and attenuation of positivity caused recruiting-like pattern. Component analysis of recruiting-like augmenting response revealed that predominant development of negativity was thought to be the result of the synchronization of preceding evoked Ⅳ-N and present Ⅱ-N or Ⅲ-N wave. And attenuation of Ⅱ-P wave was thought to be the result of the desynchronization of preceding Ⅳ-N and present Ⅱ-P wave. Responses following lower frequency stimulation with 4Hz or less were similar to responses following single stimulation. Subthreshold stimulation were thought to be difficult to induce any growth of negative cortical response. 6. It was clarified that cortical evoked response following stimulation of the VL nucleus was influenced by components and phases of the preceding cortical response, suggesting that human specific thalamic system would play a possible important role in modulation upon cortical electrical activity.