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Moist diets with increasing amounts of carbohydrate (0.5%, 10% and 21% on a dry weight basis) were each fed to duplicate groups of cod (initial weight 370 g) for 8 weeks, after which all groups were fasted for 4 weeks. Protein energy was high and accounted for more than 70% of the gross energy content in all feeds, and the diets were maintained isocaloric by substituting lipid energy for carbohydrate energy. No indigestible binder was added. Excellent growth and feed conversion were obtained in all groups. After 4 weeks of fasting, fish previously fed diets with either 10% or 21% carbohydrate showed significantly higher weight loss than fish fed the diet without carbohydrate. Liver glycogen reached 10% of liver wet weight in fish fed diets containing 10% or 21% carbohydrate and 5% in fish receiving 0.5% carbohydrate after 8 weeks. Following 4 weeks of fasting, liver glycogen was reduced to similar levels in all fish. Plasma glucose levels 4 h after feeding were higher in fish fed the diets with 10% or 21% carbohydrate and plasma free amino acid levels (FAA) were lower, than in fish fed the diet containing 0.5% carbohydrate. Blood lactate concentrations were unaffected during the first 24 h after feeding. After 4 weeks of food deprivation, the levels were significantly reduced only in the 21% carbohydrate group. A link between glucagon and protein metabolism is suggested because plasma glucagon concentration followed the same pattern as the concentrations of plasma FAA throughout the study. Insulin and glucagon-like peptide (GLP) showed a covariation throughout the experiment. Reduced plasma insulin levels were seen after fasting concomitant with reduction in the levels of FAA and glucose. It is suggested that insulin secretion in cod is affected both by plasma FAA and glucose and that cod meets food deprivation by slowing down metabolism.