Biliopancreatic bypass for obesity entails a 2/3 distal gastrectomy with Roux-en-Y reconstruction, being the small bowel transected at its midpoint and the enteroenteroanastomosis place 50 cm proximal to the ileocecal valve. Neurotensin and enteroglucagon fasting and meal-stimulated plasma concentrations were determined in 13 nonobese healthy volunteers, in 13 nonoperated obese patients, in 11 subjects within two months, in 12 subjects four to twelve months and in 7 subjects fifteen to twenty months after operation. Basal plasma enteroglucagon was significantly higher in the obese group than in the controls. However, there was no difference in the peak response, and a decrease, though not statistically significant, was seen in the integrated response. All three values were strikingly augmented in the 0-2 month group, with a highly significant difference from the preoperative group. The 4-12 and 15-20 month groups, in comparison with the 0-2 month group, showed no changes in fasting levels, a clear-cut decreased peak response and a sharp progressive reduction in integrated response, mean value in the 15-20 month group being significantly lower than that of 0-2 month group. Neurotensin basal and meal-stimulated peak plasma concentrations in the obese group were significantly higher than in the control group, whilst the integrated response was almost identical in the two groups. In postoperative groups no substantial changes in fasting levels and an increase in the peak response were observed, with a considerable progressive rise in the integrated response.