Feulgen-DNA content has been measured cytophotometrically in granule and Purkinje cells of the rat and mouse cerebellum. The study has confirmed that under normal conditions a very small part of the P. cell population in rats (less than 3%) possesses a Feulgen-DNA surplus ranging from 2C to 4C. In mice, the hyperdiploid (H2C) P. cells are even more rare. The occurrence of H2C-P. cells in rats and probably also in mice has not been substantially changed in animals exposed to factors interfering with chromatin structure and/or its template activity; the H2C-P. cells seem to be slightly more frequent after injecting mice with corticoids (Urbason) or in animals suffering from ectromelia or hereditary Purkinje cell degeneration. The incidence of H2C-P. cells has neither been substantially affected by experimental conditions which are known to lead to functional and/or metabolic stimulation of the cerebellum. Functional changes in the number of H2C-P. cell nuclei may, however, be short-term or transient in character and therefore might have excaped detection in our models. The findings rule out an impact of some reasons suspected for the artefactual origin of Feulgen H2C DNA values as e.g. compactness of the chromatin.