The effect of monosodium glutamate (MSG) on the concentration of ribonucleic acid (RNA) in the neurons of mice was studied, using the specific cytochemical stain, azure B bromide. The RNA-rich sites such as the nucleolus and the Nissl substances of large neurons showed a marked decrease in the concentration of RNA in the MSG-treated as compared to the control animals. Since RNA is believed to be the principal macromolecule involved in the learning and behavioral processes, previous reports have attributed learning and behavioral dysfunctions in MSG-treated animals to a significant decrease of the RNA concentration in these animals.