Selenium is an important nutritional element that is required in a minute amount for maintenance of proper health in both humans and animals. Many biochemical processes in human and animal depends on Se and selenoprotein function, and various studies have suggested that Se supplementation can improve fundamental immune function in both humans and animals. Se status in the UK is very low; therefore, there is a need to enhance selenium intake through diet. One way of doing that is through the introduction of selenium to farmed animals. In this Ph.D. study, we fed rainbow trout with diet containing different spiked concentrations (0.8 - 8.9 μg g-1) of Se for 14 weeks. Fish were sampled every two weeks and liver, kidney, muscle, gills and whole blood were collected and processed. The first phase of this Ph.D. study focused on selenium distribution and biotransformation in tissues of rainbow trout. Three methods were developed and used to determine total selenium concentration; species' distributions in tissues and Se peptide sequence to determine the possible incorporation of Se into proteins respectively. In the first method total selenium concentration was determined using ICP-MS and the second and third methods using HPLC-ICP-MS/ESI-MS. Total selenium concentrations in trout tissues were determined, and the highest selenium concentrations were found in liver followed by kidney, gills, and muscle. SeMet and SeCys were found to be the major species in all tissues followed by inorganic and other unknown species. To determine the possibility of Se incorporation in to protein, peptide de novo sequencing was carried out. Few selenoprotein were identified using automated de novo sequence and database search. Se species distributions in tissues of beached Pilot whales were studied. Several low molecular weights Se species were identified with selenite pre-dominating other species in most of the adult whales tissues analysed.