Background Roux-en-Y gastric bypass (RYGB) surgery causes profound changes in secretion of gastrointestinal hormones and glucose metabolism. We present a detailed analysis of the early hormone changes after RYGB in response to three different oral test meals designed to provide this information without causing side effects (such as dumping). Methods We examined eight obese non-diabetic patients before and within 2 weeks after RYGB. On separate days, oral glucose tolerance tests (25 or 50 g glucose dissolved in 200 mL of water) and a liquid mixed meal test (200 mL 300 kcal) were performed. We measured fasting and postprandial glucose, insulin, C-peptide, glucagon, total and intact glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-2 (GLP-2), peptide YY 3-36 (PYY), cholecystokinin (CCK), total and active ghrelin, gastrin, somatostatin, pancreatic polypeptide (PP), amylin, leptin, free fatty acids (FFA), and registered postprandial dumping. Insulin sensitivity was measured by homeostasis model assessment of insulin resistance. Results Fasting glucose, insulin, ghrelin, and PYY were significantly decreased and FFA was elevated postoperatively. Insulin sensitivity increased after surgery. The postprandial response increased for C-peptide, GLP-1, GLP-2, PYY, CCK, and glucagon (in response to the mixed meal) and decreased for total and active ghrelin, leptin, and gastrin, but were unchanged for GIP, amylin, PP, and somatostatin after surgery. Dumping symptoms did not differ before and after the operation or between the tests. Conclusions Within 2 weeks after RYGB, we found an increase in insulin secretion and insulin sensitivity. Responses of appetite-regulating intestinal hormones changed dramatically, all in the direction of reducing hunger.