The telencephalic proliferative response has been studied in adult newts after lesion on the central nervous system. Both in the animals injured on the optic tectum and in those on the telencephalon a telencephalic proliferative response was observed, localized in the dorsal pallial areas and in the striatum. Quantitative analysis of the proliferative activity of the matrix cells present in the telencephalon showed a different response pattern. In the animals lesioned on the telencephalon the labelled cells are much more numerous in the injured hemisphere than in the healthy one and these cells persists at a high level even in the specimens fixed 90 days after the lesion. In this case no labelled cells are observed in the midbrain. The number of labelled cells in the animals lesioned on the optic tectum was identical in both telencephalic hemispheres; this number decreases from 15 to 30 days after the injury and at 90 days labelled cells are not observed. These data confirm the presence of a neuronotrophic factor released in the ependymal fluid and active on the telencephalic periventricular matrix cells but not on the undifferentiated cells of the grey layers of the optic tectum. It is also assumed that the proliferation of matrix cells of an injured area causes the formation of glial cells and neurons, such proliferation in an intact zone is oriented to differentiation of glial cells alone.