All the available numerical information on reduction of neurones with advancing age—20 papers in all from 1894 to 1972—is critically reviewed. Only two studies (from the same laboratory) deal specifically with the human cerebral cortex: they show a steady, progressive loss of neurones throughout adult life. Of the remaining 18 studies, eight more concern man. Two deal with the cerebellum and give conflicting results. The rest concern discrete nuclei or ganglia: the effects of age are again inconsistent, but in the majority there was no reduction of neurones with advancing age. The remaining 10 papers concern animals (rodents, five; insects, three; ape, one and amphibia, one). Only two deal with the cerebral cortex as such. No consistent pattern of neurone loss emerged from these animal studies. The relevance of the data reviewed to the problem of human cortical neurone fall-out in human dementia is discussed.