The main objective of this PhD was to provide fundamental information regarding some metabolic aspects of selenium metabolism in cats and dogs. The total selenium content of a range of commercially available petfoods was analysed using a fluorometric method. The petfoods contained a wide range of selenium concentrations, with up to 6 µg Se/g DM found in cat foods. Mean concentrations of selenium in dog and cat foods were 0.40 and 1.14 µg Se/g DM respectively. All petfoods analysed met the recommended current minimum dietary selenium requirements. The use of blood parameters for the assessment of selenium status was investigated in a study in which cats were fed inorganic and organic selenium supplemented at concentrations of up to 2.0 µg Se/g DM for 32 days. Plasma selenium concentrations reflected dietary selenium intakes, however there were no differences between the different levels of supplementation. Whole blood selenium concentrations showed less distinct patterns and were thought to be a more useful indicator of longer term selenium status. Activities of glutathione peroxidase in plasma and whole blood showed no response and the response of cats to supplementation of the different forms of selenium were similar. In the same study, faecal and urinary excretion (µg/kg BW/d) were measured and apparent absorption and retention were estimated during the last seven days of the 32 day trial. Faecal excretion of selenium remained constant whereas urinary excretion of selenium increased with increased dietary intake. The form of selenium had no effect on excretion or apparent absorption however there was a trend in which more selenium was retained in cats fed organic selenium. A study was conducted with cats and dogs fed high levels (10 µg Se/g DM) of inorganic and organic selenium for 21 days to determine whether there were species differences in their metabolic response. Cats and dogs exhibited the same pattern of response, however cats showed higher plasma selenium levels, lower levels in liver and excreted more selenium compared to dogs. It was concluded from this data that cats and dogs differ in their metabolism of selenium. The effect of heat processing on the addition of inorganic and organic selenium to petfoods was investigated in cats fed 3.0 µg Se/g DM for 11 days. Apparent absorption was higher in cats fed inorganic selenium added after processing, whilst less selenium of organic origin was excreted in the urine when added after processing. These preliminary results suggest heat processing may decrease the apparent availability and utilisation of selenium in petfoods.