Total immunoreactive glucagon (IRG) and immunoreactive glucagon of A cell origin (IRGa) were measured in the serum of normal, sham-operated and depancreatized rats, after the administration of three glucagon antagonists: insulin (5--200 mU/rat/h), somatostatin (SRIF; 100 microgram/kg/h) and antiglucagon serum (AGS, enough to bind three times the calculated total amount of circulating IRG). Since no differences were noted between the responses of normal and sham-operated animals, the values were pooled and used as controls. Pancreatectomy caused a significant increase in serum glucose, IRGa and total IRG and a significant decrease in serum insulin. AGS and SRIF significantly decreased serum glucose in control, but not in depancreatized rats, even though SRIF caused a significant decrease of IRGa in all animals. SRIF significantly decreased plasma insulin in control rats, but did not modify total IRG secretion in either groups. In control rats the minimum effective hypoglycaemic dose of insulin (5 mU/rat/h) may have decreased serum IRGa, but not total IRG. At higher doses (20 mU/rat/h) insulin stimulated glucagon secretion. In depancreatized animals, higher doses of insulin (200 mU/rat/h) were needed to lower serum glucose. On the other hand, a dose of 100 muU/rat/h was sufficient to lower the serum IRG. We conclude that although hyperglucagonaemia may contribute to the hyperglycaemia of the untreated depancreatized rats, the excessive secretion of glucagon is secondary to insulin insufficiency and that, at least in this animal model, the hypoglycaemic action of insulin is only minimally dependent upon its ability to suppress glucagon secretion.