Many digestive complaints are associated with abnormalities in gastrointestinal peptide hormone function. To investigate the effect of obesity on the release of pancreatic peptide hormones, we have compared the release of insulin and glucagon in non-obese-obese Dutch women in response to isocaloric mixed meals and to Naloxone, an opioid antagonist. Healthy premenopausal women who were separated into three groups based on body mass index (BMI < 23; 23–27, > 28), were fed 600-calorie breakfasts. Higher fasting levels of plasma insulin and glucagon occurred in obese (BMI > 28) than lean (BMI < 23) women, while glucagon and insulin release after a high fat meal occurred in obese women. Naloxone administration in obese women decreased plasma insulin and glucagon, but in lean women, naloxone increased plasma glucagon but did not alter plasma insulin levels. Results indicate differences in opiate effects on pancreatic function in non-obese-obese women.