The metabolic effects of starvation and refeeding were investigated in two species of fish, the cod (Gadus morua L.) and the rainbow trout (Salmo gairdneri R.). Several metabolic parameters were measured including the levels of tissue energy reserves, blood metabolites, tissue RNA and DNA, cortisol and the activities of gluconeogenic and glycolytic enzymes in the liver and muscle. Preliminary experiments were carried out to establish the optimum assay conditions for these enzymes. Their distribution in different tissues of the cod were also studied. A study of the effect of long-term starvation (22 weeks) on metabolism of the cod revealed that the main energy reserves were mobilised in a distinct sequence. During this time there was a significant fall in circulating glucose and non-esterified fatty acids but little change in the levels of ketone bodies, glycerol, protein or amino acids. There were significant changes in the activities of some liver and muscle enzymes. An overcompensation was observed in the levels of glycogen in the liver, red muscle and white muscle in cod refed a low fat diet after 107 days of starvation. The "overshoot" in red and white muscle glycogen coincided with elevated levels of protein synthesis in these tissues and was accompanied by significant changes in the activities of some liver enzymes and in the levels of blood metabolites. Similar changes were noted when cod were refed on a high fat diet. Re-feeding rainbow trout for 8 weeks after an 8 week fast resulted in similar changes in the level of glycogen in liver and white muscle but not in red muscle. The results of these experiments were related to seasonal changes in the metabolisms of these species, especially those which are likely to influence their suitability as food.