Hypoxic hypoxia is used as one of the models for examining the unfavourable effect of oxygen deficit on the nervous system. Since some morphological studies concerning the effect of hypoxia on the central nervous system have given consideration to perivascular lesion zones and glia alterations in the white matter, the present evaluation of the effect of hypoxia has been directed to vascular endothelia of the white matter. Examinations were conducted on adult rats of the Wistar strain which were exposed to acute hypoxia by being placed in a gas mixture containing 2% O2 or to moderate hypoxia - in a gas mixture containing 7% O2. Experimental animals were sacrificed 4 and 24 h after hypoxia and 14 and 60 d after hypoxia. The behaviour of vascular endothelia after hypoxia was evaluated by karyometric and cytophotometric methods. The measurements show an edema of endothelial nuclei in capillaries of the cerebral white matter of the rat in the first 24 h after hypoxia accompanied by nuclear chromatin index alterations proportional to the intensity of hypoxia. The measurements of endothelial nuclei 14 and 60 d after hypoxia show a development of a degenerative process in these nuclei, manifested by their decreasing size and by nuclear chromatin concentration in their central parts. The morphometric and cytophotometric examinations point to long-term alterations induced by hypoxic hypoxia.