A serial blood sampling technique involving heart cannulation was used to study the effects of bovine insulin and glucagon, codfish insulin, adrenalin, noradrenalin, and glucose loading on blood glucose, plasma amino acid nitrogen, and cholesterol in the northern pike, Esox lucius. Bovine insulin at 10, 25 and 50 IU/kg produced hypoglycaemia and death 24–72 hr after injection. At 2 IU/kg, however, a transient hypoglycaemia was observed with return to normal levels after 24 hr. Codfish insulin at 2 IU/kg produced hypoglycaemia which persisted for up to 7 days. Bovine and codfish insulin lowered plasma amino acid nitrogen significantly, although return to normality was observed only at 2 IU/kg. Neither codfish nor bovine insulin alone altered plasma cholesterol levels. Glucagon at 1 mg/kg and 2 mg/kg produced hyperglycaemia at 0.5 hr which persisted for 9 hr before return to normal levels. Glucagon had no effect on either plasma amino acid nitrogen or cholesterol. Adrenalin at 0.05 mg/kg, 1 mg/kg, and 5 mg/kg produced a significant hyperglycaemia at 0.5 hr, a return to normal levels after 6 hr, and a progressive hypoglycaemia thereafter. Adrenalin at the above dose rates was generally without effect on either plasma amino acid nitrogen or cholesterol, although at 5 mg/kg, a progressive lowering of cholesterol was observed. Noradrenalin at 1 mg/kg resulted in hyperglycaemia followed by a significant hypoglycaemia after 24 hr. It was without effect on either plasma amino acid nitrogen or cholesterol. A single glucose load of 0.5 g/kg resulted in an increase in blood glucose followed by a slow return to normal levels after 48 hr, whilst the levels of amino acid nitrogen and cholesterol were unaffected. Neither the shape of the glucose-loading curve nor the levels of metabolites were significantly altered by 4 weeks of starvation. Bovine insulin (10 IU/kg) and glucose (0.5 g/kg), when injected simultaneously, resulted in a greatly improved glucose tolerance with a reduction, and subsequent recovery, of plasma amino acid nitrogen. Levels of plasma cholesterol were significantly lowered 48 hr after a combined insulin and glucose injection. These results suggest that insulin and glucagon control the circulating levels of glucose in the blood. In addition to glucagon, the catecholamines are also able to effect a rapid glycogenolysis, possibly providing for increased glucose supplies during periods of predatory activity. The influence of insulin on amino acid nitrogen in the pike may reflect an important role in this and other wholly carnivorous forms.