The total transfer RNA (tRNA) level in the liver, kidney, skeletal muscle and heart muscle of developing rats was determined by purification using (³H)tRNA as an internal standard. Liver and kidney contained almost twice as much tRNA per gram tissue as heart and skeletal muscle. There were no apparent differences between the sexes. The aminoacylation capacities of six tRNA species (alanyl, aspartyl, leucyl, methionyl, phenylalanyl, and tryptophanyl) from rat liver were not different during three developmental stages (suckling, weanling and young adult), and there were also no differences noted between males and females. The <i>in vivo</i> percent aminoacylation of four tRNAs (aspartyl, leucyl, methionyl, and phenylalanyl) was lower during the newborn and suckling periods than in weanling and young adult rat livers. The tRNA of young adults was almost completely aminoacylated <i>in vivo</i> with the exception of alanyl-tRNA.