Electron microscopy was employed to study neurofibrillary++ alterations in some parts of the brain of man aged 73 and 83 years (physiological ageing) and 36 to 80 years (vascular pathology) as well as in the spinal ganglion of dogs at different times after ischemia. Neurofibrillary++ alterations consisting in hyperplasia of microfilaments and formation of fibrillar bundles were recorded in both cases in nerve and glial cells and in their processes whereas in the human brain, they could be also detected in the intercellular space. Experimentally, filamentous hyperplasia in neurons was the most prevalent form whose intensity was related to the time elapsed after ischemia, attaining maximum in neurons with destructive processes. In human brain, filamentous alterations were not a permanent characteristic of neurons even on ageing. The greatest pleomorphism of fibrillar alterations could be observed in the soma of glial cells, in the processes of neurons and glia. Special emphasis should be laid on fibrillar bundles demonstrable in the intercellular space of human brain. Ultrastructure of the bundles was noted to be pleomorphic, which may be determined by their inconclusive genesis. It should be noted that fibrillar alterations in human brain are variable not only from case to case but also within the range of the same age. However, they are most remarkable in elderly and senile persons.