T o study the interaction between insulin secretion and insulin action in maintaining glucose homeostasis, we induced experimental insulin resistance in eight normal baboons, in six baboons treated with 40 mg/kg streptozocin (STZ-40), and in six baboons treated with 200 mg/kg streptozocin (STZ-200). Insulin resistance was induced by a 20-d continuous intravenous infusion of nicotinic acid (NA). Normal animals showed compensatory increases in several measures of insulin secretion (fasting insulin [FI], acute insulin response to arginine [AIRarg], acute insulin response to glucose [AIRgluc], and glucose potentiation slope [delta AIRarg/delta G]), with no net change in fasting plasma glucose (FPG) or glycosylated hemoglobin (HbAtc). STZ-40 animals showed compensatory increases in FI, AIRarg, and AIRgluc, but delta AIRarg/delta G failed to compensate. Although FPG remained normal in this group during NA infusion, HbA1c rose significantly. STZ-200 animals failed to show compensatory changes in both AIRgluc and delta AIRarg/delta G, with both HbA1c and FPG rising. These animals showed a paradoxical inhibition of insulin secretion in response to intravenous glucose during NA infusion, at a time when they were hyperglycemic. These data indicate that a significant degree of insulin resistance does not cause hyperglycemia in the presence of normal B cell function but, in animals with reduced B cell mass and superimposed insulin resistance, the degree of hyperglycemia is proportional to the degree of pancreatic B cell dysfunction.