Electrical low frequency stimulation (1-20 c/s) of ventrolateral (VL) nucleus of the thalamus was performed with a bipolar stimulating electrode of 0.5-1 mm distance on 25 cases of Parkinsonism. The evoked potentials were recorded with monopolar and bipolar scalp leads. The records could be averaged directly with ATAC-401 or MC-401 medical data processing computer, which had been used for only monopolar recording. The evoked potentials were noted mostly on the ispilateral side of fronto-parietal region at the regular suprathreshold stimulation, but bilateral evoked potentials with the highest amplitude of the ipsilateral fronto-parietal region at the excessive suprathreshold stimulation. EEG of fronto-parietal region was analysed before, during and after the VL stimulation by means of Walter-type hand-pass filter apparatus. It was interesting that waxing and waning phenomena were observed in the analysed band, which was concordant with the frequency of stimulation. When 6 c/s stimulation was applied, the phenomena were obtained clearly in the θ band, 10 c/s in the α band. The evoked potentials were assumed as an augmenting response when 5-10 c/s repetitive stimulation was applied to VL nucleus. The averaged response showed small primary positive-negative-positive phases within 30 msec after stimulation, followed by large late negative phase with 50 to 60 msec of peak latencies. But in some cases, the responses averaged with the computer, showed primary positive phase with 5 to 10 msec and a following large negative phase with 30 to 40 msec of peak latencies, which led into a more variable positive wave with peak latency 50-65 msec. The peak latencies of both positive and negative phases showed a fairly constant duration in the stimulations of lower frequencies (1-10 c/s). The large negativity with peak latency 30-60 msec showed maximum amplitude at 8-10 c/s stimulation. When the stimulations of higher frequencies were appiled, the large negativity became lower and lower in amplitude and finally disappeared at the frequency of 15-20 c/s. It was considered that the large negativity was the most principal component in the augmenting response of VL stimulation. However, in five cases of parkinsonian patients who showed typical augmenting responses at 5-10 c/s stimulation, recruiting-like responses were also noted at 15-20 c/s. The position of the tip of the stimulating electrode was checked roentgenologically, and identified to be in the regular VL area of the thalamus. It could be considered that ventrolateral nucleus of the human thalamus has some aspect of diffuse projecting systems in it or close relation to them electrophysiologically.