We have recently reported that the Xenopus laevis complementary DNA (cDNA), XL-15, encodes a selenoprotein that is a 5-deiodinase (5D). XL-15 represents a gene that is up-regulated by thyroid hormone in this species. We now report the isolation from a Rana catesbeiana (RC) cDNA library of a cDNA for the RC 5D (RC5D). RC5D a 1534-base pair cDNA that exhibits 78% identity to XL-15 in the coding region, including the TGA codon, which in XL-15 encodes selenocysteine, and a putative selenocysteine insertion sequence (SECIS) located in the 3'-untranslated region. Transcripts of RC5D, synthesized in vitro, induce 5D activity after their injection into Xenopus laevis oocytes. RC5D hybridizes to a 2.2-kilobase messenger RNA (mRNA) species in RC tissues. The expression of RC5D was studied in 10 tissues from untreated and T3-treated premetamorphic tadpoles. RC5D mRNA transcripts and 5D activity were detected in most tissues examined, and the levels of both were greatly increased in T3-stimulated tadpoles. Furthermore, the levels of RC5D mRNA transcripts correlated closely with 5D, but not with 5'-deiodinase (5'D), activity. 5'D was not enhanced by a 4-day exposure of the tadpoles to T3. The up-regulation of RC5D gene expression by T3 was demonstrable in tadpoles by stage V, but was found to be transient. The levels of RC5D transcripts and 5D activity were highest after 2-3 days and 5 days of T3 exposure, respectively, but had fallen by 10 days to levels comparable to those in unexposed tadpoles. It is concluded that RC5D encodes a selenoprotein that is a 5D, and it represents a gene that is up-regulated by T3. On the basis of the findings presented herein, it is suggested that the 5D system plays a major role in regulating intracellular T3 levels in developing tadpoles.