Histochemical and immunohistological examinations were carried out at different stages of thiamine-deficient encephalopathy in rats. The respiratory enzymatic activity decreased in the most damaged area correlating well with the neuropathological findings. There was an inverse relationship between the damaged area and its marginal zone; the latter showed an increase of the same enzymatic activity. The capillary network and the activity of the vessel walls seemed to be almost unimpaired. The immunohistological investigations showed only a moderate extravasation of the plasma proteins.