Cells found throughout the lining of the gastrointestinal tract that contain regulatory peptide hormones and/or biogenic amines. The substances are located in secretory granules and act in an endocrine or paracrine manner. Some of these substances are also found in neurons in the gut. There are at least 15 different types of endocrine cells of the gut. Some take up amine precursors and have been called APUD CELLS. However, most endocrine cells of the gut apparently have endodermal rather than neuroectodermal origin, so the relationship with APUD cells is not clear.