The mechanism investigators propose to account for success is reduction in food intake brought about by the pattern of hormone release, particularly glucagon-like peptide-1 (GLP-1), and gastric distention, both known to underlie satiation, coupled with post-ingestive changes in reinforcing value of food and motivation to consume. Tests of the hypothesis will be done by measuring responses to tasting, working for, and consuming, foods on sensory, motivational, cognitive, and physiological variables, including amount consumed and rate of eating under standardized conditions, before surgery and at a two year follow up visit. In addition the inhibitor exendin-9-39 (EX9) will be infused before a meal to determine whether blockade of GLP-1 Receptor after surgery partially reverses intake reduction. Investigators predict that successful patients will show changes that favor reduction in food intake, rate of eating, motivation to consume, reward value of foods, and a hormone profile that has been shown to generate satiation and maintain reduction in intake (e.g. increased Cholecystokinin (CCK), GLP-1, reduced ghrelin). To the extent that psycho-social and cognitive factors may override physiological brakes to eating behavior, the subjects may fail to achieve success, and consequently the failure may be predicted from over-impulsiveness or inability to suppress working for rewarding food stimuli. To test these hypotheses, a total of 50 patients will be enrolled prior to RYGBP and restudied 2 years after the surgery. Based on known high attrition in this population and our past 10 year experience with bariatric research, it is estimated that 100 patients will need to be enrolled over 2 years to obtain 50 who complete testing at 2 years. EX9 studies will be done in a subset of 30 completers, only after roux-en-y gastric bypass (RYGBP), 15 failure (<15% weight loss) 15 success ( >30% weight loss).