The density and size of astrocyte, oligodendrocyte and neurone nuclei were determined in the corpus striatum of rats with urease-induced hyperammoniemia (plasma NH4+-concentration about 800 micronmol/1). No changes in the number of neurone nuclei were found. After 4 days the density of astrocyte nuclei increased from 13 to 23 per cent of the glial nuclei. However, the number of oligodendroxyte nuclei decreased correspondingly and the total glial nuclear number remained constant. The number of astrocyte nuclei was normalized 1 week after the 4-day period of hyperammoniemia. Moreover, intracerebral injections of colchicine at different times of the experimental period revealed no mitoses, indicating that no astrocyte divisions took place in pure hyperammoniemia. The astrocyte nuclei were of normal size in the urease animals in contrast to the animals with porto-caval anastomosis (PCA) which showed enlarged astrocyte nuclei. Thus, hyperammoniemia caused a reversible transformation of glial nuclei, but no real proliferation. A comparison of the glial reactions 4 days after a brain lesion showed the same frequency of astrocyte mitoses in control and urease animlas but a higher incidence of mitoses in the PCA animals. The number of Alzheimer type I astrocytes was the same in control and PCA animals, whereas no such cells were found in the urease animals, indicating that this form of hyperammoniemia did not lead to arrest of astrocyte metaphases with subsequent formation of Alzheimer type 1 cells.