during a short period of induced protein synthesis could be blocked by actinomycin D. It was concluded, therefore, that the RNA was immediately DNA dependent and directly related to the genetic apparatus. Rats which were normally right handed were forced to modify their handedness in order to obtain food. The RNA of nerve cells in that part of the cortex, whose destruction destroys the ability to transfer handedness, was analyzed. A significant increase in RNA of nerve cells of the fifth to sixth cortical layers on the right side of the brain was observed. The corresponding nerve cells on the opposite side of the same brain served as controls. There was an increase in RNA and a significant increase in the purine bases relative to the pyrimidine bases in the learning side of the cortex. When the animals were not forced to learn a new procedure, only an increase of RNA was observed, with no change in base ratio. Frank Morrell, head of the Neurology Department at Stanford Medical School, has also been active in this field during the past 6 years. He has found that if a primary epileptic lesion is induced on one side of the cortex, a secondary mirror lesion eventually develops in the contralateral homologous cortex. This secondary lesion, which showed self-sustaining epileptiform discharge, could be isolated, whereupon the epileptiform discharge disappeared. This was interpreted as learned behavior of the secondary lesion. From changes in the staining properties of the secondary lesion, Morrell concluded that changes in RNA had occurred in the cell. Changes in the composition of the RNA could not be shown by these techniques. At the University of California at Berkeley, Drs. Rosenzweig, Bennett, and Krech have conducted extensive studies related to this topic. These investigators have directed their efforts toward demonstrating alterations in the cerebral cortex of animals exposed to continuing learning situations or continuously deprived of sensory stimulation. In a recent publication ([ref.116]), which also summarizes a considerable amount of previous work, they report studies which demonstrate the following: (1) Rats given enriched experience develop, in comparison with their restricted littermates, greater weight and thickness of cortical tissue and an associated proportional increase in total acetylcholinesterase activity of the cortex. (2) The gain in weight of cortical tissue is relatively larger than the increase in enzymatic activity. Acetylcholinesterase activity increases in other portions of the brain even though tissue weight decreases. (3) The changes appear in a variety of lines of rats, although differing in amount between strains. (4) The changes are observed in both the young and adult animals.