Although it has long been known that opioid peptides cause marked changes of pituitary hormone secretion in both animals and humans, little is known about the possible effect(s) of food-derived opioids (exorphins) on pituitary function. In order to investigate the possible role of exorphins derived from wheat gluten on pituitary function, we gave the following treatments to four groups of male rats: intracerebroventricular (ICV) vehicle, Gluten Exorphin B5 (GE-B5) 200 microg ICV, naloxone intraperitoneally (IP) followed by vehicle ICV, naloxone IP followed by GE-B5 ICV. Blood samples for Prolactin (PRL) and Growth Hormone (GH) were taken at intervals for 90 minutes after vehicle or GE-B5 administration. GE-B5 strongly stimulated PRL secretion; its effect was completely abolished by naloxone administration. GH secretion was unaffected by GE-B5 under these experimental conditions. The present study shows for the first time that an opioid peptide derived from wheat gluten, GE-B5, has an effect on pituitary function when administered ICV; its mechanism of action appears to be mediated via classical opioid receptors.