Carbohydrate-rich diets are frequently used in the commercial farming of fish. However, fish do not appear to have an elevated capacity to clear a high load of glucose from their circulation. Special attention has been directed towards the study of the physiological responses to such carbohydrate-rich feeding. Skeletal muscles play a key role in glucose homeostasis in mammals, and exercise has a noticeable effect in lowering plasma glucose. This effect is caused by an up-regulation of glucose transport mediated by GLUT4 in skeletal muscles in response to exercise. However, the exact mediating mechanism in the modulation of glucose transport in skeletal muscle has not been entirely characterized in vertebrates. Moreover, fish are an excellent model for the research of mechanisms involved in changes of the GLUT glucose transporters produced by exercise because they have anatomically separated red and white muscle masses, which allow the measurement of fibre-type-specific responses. I hypothesize that sustained exercise can be used to enhance carbohydrate expenditure in fish, increasing their natural capacity to clear and utilize plasma glucose by the contracting muscle. I will investigate this possibility, placing a special emphasis on the study of the cellular and molecular mechanisms regulating the GLUT-1 and -4 glucose transporters in white and red muscles. Changes in patterns for gene expression using a microarray platform and key biochemical parameters will also be measured in red and white muscles of rainbow trout after four weeks of sustained swimming. I expect that the results obtained from this research will help us to understand how glucose is regulated in fish. This knowledge could be transferred to European companies, leading to more efficient and environmentally conscious feeding practices in the rapidly growing and economic important aquaculture sector.