Selenocysteine (Sec) tRNAs serve as carrier molecules for the biosynthesis of Sec from serine and to donate Sec to protein in response to specific UGA codons. In this study, we describe the current status of Sec tRNAs in higher animals and further we exarnine: (i) the Sec tRNA population in Drosophila; (ii) transcription of the Sec tRNA in vivo (in Xenopus oocytes) and in vitro (in Xenopus oocyte extracts); (iii) the effect of selenium on the Sec tRNA population in various rat tissues following replenishment of extremely selenium deficient rats with this element; and (iv) the biosynthesis of the modified bases on Sec tRNA in Xenopus oocytes Selenocysteine ​​(Sec) tRNAs serve as a carrier molecule for the biosynthesis of Sec from serine and to donate Sec to protein in response to specific UGA codons. In this study, we describe the current status of Sec tRNAs in higher animals and further we exarnine: ( i) the Sec tRNA population in Drosophila; (ii) transcription of the Sec tRNA in vivo (in Xenopus oocytes) and in vitro (in Xenopus oocyte extracts); (iii) the effect of selenium on the Sec tRNA population in various rat tissues following replenishment of extremely sevelen deficient rats with this element; and (iv) the biosynthesis of the modified bases on Sec tRNA in Xenopus oocytes