The purpose of the study was to examine the gastrointestinal hormone responses to meals in morbidly obese patients before and after Roux-en-Y gastric bypass (GBP; n = 9) or vertical banded gastroplasty (VBG; n = 7). On consecutive days before and after operation, we measured changes in peripheral blood levels of glucose, insulin, enteroglucagon, serotonin, vasoactive intestinal polypeptide (VIP), and cholecystokinin (CCK) in response to a standardized glucose or protein-fat meal. The percentage of excess weight lost at 6 months after operation was 66.3% +/- 4% and 41.8% +/- 5% for GBP and VBG, respectively (p less than 0.01). The 3-hour integrated glucose response to a glucose meal decreased from 145.3 +/- 33.7 to 75.8 +/- 15.7 g min/L (p less than 0.02) after GBP. This was associated with a decrease in 3-hour integrated insulin response from 22.8 +/- 8.2 to 10.5 +/- 4.9 mU min/L. Vertical banded gastroplasty patients had lesser reductions of hyperglycemia and hyperinsulinemia. Neither the CCK, serotonin, nor VIP responses to meals were altered by either operation. The 3-hour integrated enteroglucagon response to glucose increased markedly in GBP patients after operation from 11.8 +/- 6 to 133.4 +/- 38 nmol min/mL (p less than 0.02). This increase in enteroglucagon occurred at the same time as development of dumping symptoms, which occurred exclusively in GBP patients after glucose but not protein. We conclude that (1) GBP surgery for morbid obesity results in amelioration of glucose intolerance and hyperinsulinemia, (2) CCK does not mediate an endocrine satiety effect of surgery, (3) GBP is associated with an exaggerated enteroglucagon response to glucose, and (4) enteroglucagon appears to be a marker of the dumping syndrome in GBP patients.