Among the foremost investigators of the chemistry and biochemistry of the central nervous system is Holger Hyden at the University of Göteborg, Sweden. He and others (refs. [ref.118]-[ref.120]) have for many years performed elegant microanalytical studies of single nerve cells. The evidence which Hyden has obtained is consistent with the hypothesis that the initial electrical reverberations in the brain induce a change in the molecular structure of the ribonucleic acid (RNA) of the neurones which, in turn, leads to a subsequent deposition of specific proteins. It is well known from other investigations that a major role of RNA in any type of cell is to specify and mediate synthesis of the protein enzymes of the cells. Thus, in this hypothesis, it is only necessary to postulate the modification of brain RNA by the activities associated with reverberational memory. Particularly pertinent to this hypothesis are observations that— (1) Large nerve cells have a very high rate of metabolism of RNA and proteins, and, of the somatic cells, are the largest producers of RNA. (2) Vestibular stimulation by passive means leads to an increase in the RNA content of the Deiters nerve cells of rabbits ([ref.121]). The protein content of these cells is also increased. (3) Changes in the RNA composition of neurones and glia of the brainstem occur during a learning situation. Animals were trained over a period of 4 to 5 days to climb a steeply inclined wire to obtain food. The big nerve cells and the glia of their lateral vestibular apparatus were analyzed, since the Deiters neurones present in this structure are directly connected to the middle ear. The amount of RNA was found to be increased in the nerve cells; and, more significantly, the adenine-to-uracil ratio of both the nuclear RNA of nerve cells and glia cells became significantly increased ([ref.119]). A variety of control experiments were conducted. Although there was an increase in RNA content of these cells in animals exposed to passive stimulation, there was no change in the ratio of adenine to uracil. Nerve cells from the reticular formation, another portion of the brain, had only an increased content of RNA with no base-ratio change. Animals subjected to a stress experiment involving the vestibular nucleus showed only an increase in content of RNA. Littermates living in cages on the same diet as learning animals showed no change in content of RNA. Thus, it would appear that the change in the base ratio of the RNA synthesized is not due to increased neurone function per se, but is more directly related to the learning process. The fact that this was nuclear RNA implies that it was immediately related to chromosomal DNA. (4) Neuronal RNA with changed cytosine-guanine ratios synthesized