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Metabolism of total nonhistone nuclear proteins (NNP) of the rat brain was studied in three groups of animals: trained, pseudotrained and control. Protein synthesis and degradation were estimated by determining NNP specific radioactivity 1, 3, 6 and 9 weeks after the precursor, 14C-lysine, injection. The experimental results indicate that there are two fractions of NNP which differ in their metabolic rates. The half-life of the first "short-living" group of proteins is 5-10 days and that of the second "long-living" one is 18-21 days. It is shown that there is a significant difference in specific radioactivity and metabolic rate between the "short-living" fractions of NNP isolated from the brains of trained or pseudotrained rats, on the one hand, and control rats, on the other hand. Possible participation of the brain total nonhistone nuclear proteins in the mechanisms of alterations of the central nervous system function and learning is discussed.