Selenium exerts its biological activity largely through selenoproteins, which contain the element in the form of selenocysteine. Five selenoproteins have been characterized in animal tissues and there is evidence that a number of others exist. Selenoprotein synthesis is a complex process that has been well characterized in prokaryotic systems but incompletely characterized in eukaryotic systems. Selenium deficiency causes a decrease in selenoproteins, but the decrease is not uniform and some selenoproteins are maintained better than others. The selenoprotein most sensitive to selenium deficiency is liver cGSH-Px. It contains a significant fraction of the selenium in the body, and decreased synthesis of it under deficiency conditions might serve to increase the selenium available for synthesis of selenoproteins that are more important to the survival of the animal than is cGSH-Px. The regulation of individual selenoproteins in selenium deficiency appears to be at the mRNA level. Factors that affect mRNA levels have not been completely characterized, but the fall in cGSH-Px mRNA in rat liver is not accompanied by decreased transcription, which suggests that it is regulated through changes in degradation.