In the course of stereotactic thalamotomy, the neural noise level of subcortical structures was estimated quantitatively with the aid of two semimicro-electrodes. The neural noise showed several characteristic features in terms of its amplitude and discharge pattern so that it was correlated with the possible anatomical substrate, thus providing the functional anatomy of the subcortical structures. The study on saggital plane revealed a systematic difference of the noise level between VL and Vim-Vc that could be explained by the different cell size in respective nucleus. Several exceptional cases were also presented, emphasizing the neural noise study in stereotactic surgery.