This thesis focuses on the influence of selenium supply on the expression of selenoproteins and possible mechanisms involved. It examines the effects of selenium on the expression of cytosolic glutathione peroxidase (cGSH-Px), phospholipid hydroperoxide glutathione peroxidase (PHGSH-Px) and type I iodothyronine 5'deiodinase (IDI) in liver, thyroid and heart of the rat. Nutritional studies in animals fed diets with different selenium content have shown that there is differential regulation of the three mRNAs and subsequent protein synthesis within and between organs, suggesting both that mechanisms exist to channel selenium for synthesis of a particular selenoprotein and that there is tissue-specific regulation of selenoprotein mRNAs. Since selenium supply could regulate selenoprotein expression by either transcriptional or post-transcriptional mechanisms, the effects of selenium depletion on the transcription rate of the three genes and their mRNA stability were investigated. Nuclear run-off assays with isolated liver nuclei showed selenium deficiency to have no effect on transcription of the three genes whereas actinomycin D chase experiments, in hepatoma cells, showed that selenium deficiency had no effect on the stability of PHGSH-Px mRNA but decreased the stability of cGSH-Px mRNA. Additionally, studies with hepatoma cells transfected with chimeric constructs of IDI coding region linked to different 3'UTRs showed that cGSH-Px 3'UTR is less efficient than PHGSH-PX 3'UTR in permitting Se-cysteine incorporation when selenium is limiting. In conclusion, the mechanisms of selenoprotein regulation in selenium deficiency involve post-transcriptional controls: in liver, selenium deficiency affects translation of cGSH-Px and PHGSH-Px mRNAs at different extents and differences in the 3'UTR of these two mRNAs could partially explain the differential effect of selenium deficiency on cGSH-Px and PHGSH-Px activities and mRNA abundance, stability and translation.