Previous studies on cattle incubating BSE have suggested that the energy metabolism of these animals is disrupted during the preclinical stages of the disease, particularly at times associated with nutritional stress such as early lactation (Moorby et al. 2000). This disruption is manifest as increased plasma concentrations of lactate, with-hydroxybutyrate, alanine and leucine in conjunction with decreased concentrations of serine and glutamic acid (Moorby et al. 2002). The hypothesis suggested was that animals become more dependent upon glycolysis as TSE disease progresses. This hypothesis is supported by more recent studies with dairy cattle infected with BSE, that showed similar increases in plasma alanine and demonstrated decreased ratios of muscle glycogen to plasma lactate compared to healthy control cattle, suggesting that BSE infected cattle have altered energy metabolism (Allison et al. 2007). Furthermore, NMR studies have suggested that changes in energy metabolism also occur in sheep during the pre-clinical stages of scrapie infection (Charlton et al. 2006). The present study was undertaken to examine whether the TSE dependent causes of those metabolic disturbances could also affect whole body control of plasma glucose concentration. Two approaches were taken: firstly by conducting a survey of plasma glucose concentrations in populations of cattle and sheep incubating TSE disease, and secondly by undertaking an experimental study of the hyperglycaemic response to xylazine of field case BSE suspect cattle. Our results suggest that plasma glucose concentrations are maintained during TSE pathogenesis despite the disruptive affects of the disease on energy metabolism.