Ammonia is an essential intermediate of a number of metabolic pathways in the body, from the maintenance of normal brain function, efficient immune response, to the production of energy within muscle cells to maintain contraction during exercise. However, ammonia levels must be carefully maintained within a low concentration range (no greater than SO-100Ilmol/L), or toxicity may develop. Excess ammonia, also known as hyperammonaemia, has been linked to the development of neurological dysfunction in liver disease states and certain in-born genetic metabolic defects (e.g. urea cycle enzyme deficiencies), whilst it is also believed to impair the regulation of protein metabolism in a number of tissues. This has led some to speculate that similar impairment could occur in healthy individuals where hyperammonaemia is common, such as extremes of exercise, and that hyperammonaemia may be implicit to the altered protein metabolism and fatigue development during such exercise. Both disease and exercise are complicated by a multitude of other metabolic disturbances, and although many functional disturbances in these states are related to the onset of hyperammonaemia, it is difficult to identify cause and effect due solely to ammonia itself in the presence of so many other variables. The aim of this thesis is therefore to investigate the metabolic and functional consequences of hyperammonaemia from a whole body to a cellular level using novel and innovative techniques. Experiment one of this thesis describes the design and implementation of a method for inducing a tolerable and pathologically/physiologically relevant state of hyperammonaemia in resting healthy adult males. This method isolates hyperammonaemia from other variables present in disease or exercise, thereby allowing the effects of ammonia on metabolism and function to be directly assessed. Experiment two uses this method and provides evidence for some subtle changes in response to hyperammonaemia. Using a double-blind placebo controlled cross-over design, it was found that there was a significantly higher level of sensations of fatigue reported during the hyperammonaemia trial compared to the placebo trial, with an absence of impairment in psychological task performance. The mechanisms behind this increase in fatigue, due to ammonia, were investigated in experiment three using quantitative brain MRI techniques. Minimal structural or metabolic changes were identified in brain as a result of the hyperammonaemia, which reflects the lack of impairment in psychological task performance. It may therefore mean that ammonia alone has little influence on brain function, and the common psychological impairment associated with hyperammonaemia in disease is related to other co-morbidities and/or its interaction with these co-morbidities. Despite these finding, sensations of fatigue were still present following induction of hyperammonaemia, supporting the findings of the previous experiment. It was difficult to interpret what may be causing these changes in fatigue sensation, particularly with the lack of effect to MR imaging. A significant decrease in brain fractional anisotropy (FA) was observed, which could represent subtle microstructural changes in the brain related to water regulation due to ammonia, in turn influencing perceived sensations of fatigue. However, these brain FA changes were not supported by other MR measures of brain water regulation (MTR and lH-MRS), so this explanation remains speculative. Experiment four used a cell culture model to determine the effects of excess ammonia on skeletal muscle function, in the form of protein metabolism. Although brain is often the most investigated organ with regards to ammonia's affects, large concentrations are present in skeletal muscle in both exercise and disease, and therefore warrant further investigation. Findings from this study show a potent inhibition to protein synthesis, via control of mRNA translation at both initiation and elongation phases. These results will have wider impact if confirmed in vivo, as they provide new avenues for investigation with regards to the control of MPS, and may provide potential for development of new therapeutic/nutritional targets. Taken together, the findings of this thesis will have a wide ranging application in conditions where hyperammonaemia is prevalent, potentially identifying therapeutic targets for the reversal or delay in ammonia induced impairment, whilst also highlighting the need to investigate the interactive effects between ammonia and the other metabolic changes present in liver disease and exercise, in addition to its array of related functional and metabolic disturbances.