Decreased insulin sensitivity (IS) in dairy cows supports milk yield but increases the risk for metabolic and reproductive disorders. Although several inducers of decreased IS are known, it is unclear to what extent it is congenitally determined. The main aim was to investigate differences in IS between neonatal calves of the Belgian Blue (BB) breed, reared for beef production, and the Holstein-Friesian (HF) breed, reared for milk yield. Additionally, a small number of East Flemish (EF) calves, a local dual-purpose breed, were compared with the 2 other breeds. Ten BB, 12 HF, and 4 EF calves with similar age, ration, and housing were selected. In the intravenous glucose tolerance test, blood samples were taken at regular intervals after an intravenous glucose bolus of 150 mg/kg. Area under the curve (AUC), peak concentration, and elimination rate of insulin and glucose were computed. The quantitative insulin sensitivity check index (QUICKI) and revised QUICKI were computed using basal glucose, insulin, and nonesterified fatty acid concentrations. In the intravenous insulin tolerance test, blood samples were obtained from 4 calves of each breed at regular times after an intravenous insulin challenge of 0.05 IU/kg. Based on the decline in glucose concentrations relative to basal levels, the insulin-stimulated blood glucose response was computed. Basal insulin concentrations were higher in HF (1.58 +/- 0.40 mu U/mL) than in BB calves (0.35 +/- 0.09 mmol/L). Compared with BB calves, HF and EF calves had higher basal glucose concentrations (4.40 +/- 0.16 vs. 5.70 +/- 0.35 and 5.81 +/- 0.13 mmol/L, respectively), insulin peak concentrations (4.62 +/- 1.09 vs. 9.70 +/- 1.45 and 16.44 +/- 5.58 mu U/mL, respectively), insulin AUC (86.71 +/- 18.81 vs. 222.65 +/- 45.00 and 293.69 +/- 109.22 mu U/mL.min, respectively), and glucose AUC (256.22 +/- 17.53 vs. 335.66 +/- 18.74 and 321.03 +/- 10.05 mmol/L.min, respectively). Glucose elimination rates were lower in HF (1.37 +/- 0.22%/min) than in BB calves (2.35 +/- 0.25%/min). ...