Insulin secretion following the intravenous infusion of gastric inhibitory polypeptide (GIP) is diminished in patients with type 2 diabetes and at least a subgroup of their first-degree relatives at hyperglycemic clamp conditions. Therefore, we studied the effects of an intravenous bolus administration of GIP at normoglycemic conditions in the fasting state. Ten healthy control subjects were studied with an intravenous bolus administration of placebo, and of 7, 20, and 60 pmol GIP/kg body weight (BW), respectively. Forty-five first-degree relatives of patients with type 2 diabetes and 33 matched control subjects were studied with (1) a 75-g oral glucose tolerance test (OGTT) and (2) an intravenous bolus injection of 20 pmol GIP/kg BW with blood samples drawn over 30 minutes for determination of plasma glucose, insulin, C-peptide, and GIP. Statistical analysis applied repeated-measures analysis of variance (ANOVA) and Duncan's post hoc tests. Insulin secretion was stimulated after the administration of 20 and of 60 pmol GIP/kg BW in the dose-response experiments (P <.0001). GIP administration (20 pmol/kg BW) led to a significant rise of insulin and C-peptide concentrations in the first-degree relatives and control subjects (P <.0001), but there was difference between groups (P =.64 and P =.87, respectively). Also expressed as increments over baseline, no differences were apparent (Delta(insulin), 7.6 +/- 1.2 and 7.6 +/- 1.6 mU/L, P =.99; Delta(C-peptide), 0.35 +/- 0.06 and 0.38 +/- 0.08 ng/mL, P =.75). Integrated insulin and C-peptide responses after GIP administration significantly correlated with the respective insulin and C-peptide responses after glucose ingestion (insulin, r = 0.78, P <.0001; C-peptide, r = 0.35, P =.0015). We conclude that a reduced insulinotropic effect of GIP in first-degree relatives of patients with type 2 diabetes cannot be observed at euglycemia. Therefore, a reduced GIP-induced insulin secretion in patients with type 2 diabetes and their first-degree relatives at hyperglycemia is more likely due to a general defect of B-cell function than to a specific defect of the GIP action.