Food-derived bioactive peptides are regarded as important modulators of several physiological processes occurring both systemically and locally within the gastrointestinal tract (GIT). However, the concentrations of food-derived bioactive peptides in the GIT, and therefore attendant physiological effects, are likely to be highly variable given the wide variation in the type and amount of dietary protein consumed either during the day or on a day-to-day basis. In contrast, gut endogenous proteins (e.g. cell proteins, mucin, serum albumin and digestive enzymes) are a consistent and significant potential source of peptides for the GIT. With up to 80% of gut endogenous proteins being digested in the GIT, it is possible that a wide range of peptides is generated, but until now the significance of the gut endogenous proteins as a source of bioactive peptides has not been considered. A hypothesis is promulgated that the gut endogenous proteins may have a hidden role as a consistent and quantitatively important source of bioactive peptides in the GIT.