Cell proliferation in the brains of rats and cats was investigated autoradiographically. Two young adult rats were injected intraperitoneally with 2 mc of thymidine‐H³ and killed after a two weeks' exchange period. Two adult cats were injected intraventricularly with 0.5 mc of thymidine‐H³ and killed one week later. Labeling of cell nuclei in the brain, presumed to reflect DNA turnover and cellular proliferation, was investigated. In the rats, some neuroglia cells were found labeled in all parts of the brain, suggesting a low rate of glial proliferation. In addition, circumscribed small regions with numerous labeled neuroglia and microglia cells were seen in several brain regions, suggesting the occurrence of local glial proliferative reactions in these presumably normal brains. A few apparently labeled neurons were seen in the neocortex, and a proliferative region of granule cells was identified in the dentate gyrus of the hippocampus. In the cats labeling of glia cells was highest in the midline region, near the point of injection of the radiochemical, with a gradient of decreasing number of labeled cells both laterally and in the anteroposterior direction of the neuraxis. Neurons with apparently labeled nuclei were observed in the midline cortex bilaterally in both animals. These results indicate that glia cells can multiply in the brains of young adult rats and adult cats and they support the possibility that new neurons may be formed in forebrain structures, both in rodents and carnivores.