Context Roux-en-Y gastric bypass (RYGB) is associated with postprandial hyperinsulinemia. Objective This study assessed whether increased blood insulin levels may be due to an increase in maximal β -cell function. Design, Setting, and Participants We performed a cross-sectional study at Columbia University Medical Center, New York, New York. Subjects without a history of diabetes were studied after surgery (n = 12) and were compared with nonsurgical controls (n = 10) who were mean matched for body mass index, insulin sensitivity, and hemoglobin A1c and with nonobese controls (n = 8). Methods Subjects underwent a mixed-meal tolerance test and on a separate day an intravenous glucose tolerance test followed by a hyperglycemic clamp (450 mg/dL; 25 mM blood glucose) and arginine stimulation. The main outcome measure was maximal insulin secretion quantified after arginine stimulation (AinsRmax). Results The RYGB group exhibited greater peak postprandial glucose levels and fourfold greater peak insulin levels than control groups; however, there were no significant differences in insulinogenic index or AinsRmax. Another finding was significantly greater postprandial glucagon levels in the RYGB group compared with controls. Conclusions Our results suggest that after RYGB, the increase in postprandial levels of insulin are not due to changes in maximal β -cell function but appear to be an appropriate response to altered nutrient flow and absorption.