The combination of scanning electron microscopy of microcorrosion casts with dark field microscopy of the cerebral vascular system has made it possible to clarify in detail the morphology of microcirculation in the rat. It was concluded by preliminary experiments using vasospasm provoked by direct administration of vasopressin to the basilar artery that the polyester casts could clearly represent the morphological change occurring just before commencement of perfusion fixation though slight artificial modification was observed during the succeeding procedures. The architecture of the blood vessels was observed in the parietal cortex and the ventrolateral nucleus of the thalamus with special reference to structures contributing to regulation of the microcirculation. By both of the two methods the circular foldings which are supposed to be a constrictive structure were frequently observed even in arterial vessels up to 10 microns constituting the capillary plexus. Similar structure was also seen in the medium to small sized venous vessels of the regions observed. The results obtained seem to need further studies.