Glutathione peroxidase (GSH-Px) contains selenium (Se) as selenocysteine in the active site of the enzyme. GSH-Px activities in the cytosol of all guinea-pig tissues examined were extremely low compared with those in mice and rats, while Se concentrations in tissues were almost the same among three animal species. In addition, no GSH-Px mRNA was detectable in any tissues of guinea-pigs, although the guinea-pig had the same copy number (probably a single copy) of the GSH-Px gene in its genomic DNA as that of the mouse and rat, suggesting that the species difference of GSH-Px activity observed in rodents might be due to incapability of gene transcription. On the other hand, feeding of mice with Se-deficient diet for 6 weeks resulted in a remarkable decrease in GSH-Px mRNA as well as GSH-Px activity both in the liver and kidneys. The detailed time-course experiment revealed that the drop in GSH-Px activity preceded the decrease in the mRNA level in Se-depleted mice and the mRNA level recovered rapidly in contrast to the slow rate of increase in the enzyme activity in Se-repleted mice. These results suggested that the alteration in GSH-Px activity in mice subjected to dietary Se manipulation is attributable not only to transcriptional but also to post-transcriptional regulation.