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Abstarct –The total tRNA and some specific tRNAs from the 100,000g soluble fraction of rat brain were measured during development (postnatal ages 4–55 days). For determination of specific tRNAs we developed a method that measured their capacities to bind specific amino acids. Levels of total tRNA were decreased in the soluble fraction from the brains of 55‐day‐old rats in comparison to those for the 4‐day‐old rats. The aminoacylation capacities of tRNAs for phenylalanine, lysine, proline, valine, leucine, alanine and isoleucine were diminished in the 55‐day‐old rats in comparison to those for 4‐day‐old rats when expressed per unit wet weight of brain. When the 4‐ to 55‐day changes in aminoacylation capacity of each specific tRNA was expressed relative to that of the total tRNA, tRNAPhe and tRNALysLys were diminished; tRNAPro, tRNAVel, tRNAGIY and tRNALeu showed no significant changes; and tRNAA1a and tRNAIle were increased. Incorporation of amino acids into a material insoluble in hot TCA (probably proteins) in a ribosome‐free system occurred in the brain preparations. Out of ten different amino acids studied, arginine and tyrosine exhibited the highest values for this type of transfer.

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Rats kept in the dark (i.e., less than 0.1 lux) from birth until 15 days were exposed to light. In vivo incorporation of 14C-leucine into acid insoluble material and in vitro aminoacylation of tRNAleu were studied in visual, somaesthetic and motor brain cortices. A significant increase of both 14C-leucine incorporation into acid insoluble material and tRNAleu aminoacylation with respect to the controls were found only in the visual cortex of light stimulated animals.