Recent investigations have shown that the functions of the gastrointestinal organs may be regulated by a complex system of paracrine cells and peptidergic nerves, in addition to the established humoral and autonomic nervous systems. In this article we discuss the possible paracrine regulation of the secretory functions of the stomach and the pancreas by somatostatin-producing D-cells. Secondly, we discuss the distribution, localization, effects and secretion of the neuropeptides VIP and 'gastrin-releasing polypeptide' (GRP or 'mammalian bombesin') applied to the stomach (GRP) and pancreas (GRP and VIP) of the pig. It is concluded, that all three peptides are capable of influencing powerfully the secretory state of these organs, and that these newer regulatory peptides probably play an important role in the integrate neurohormonal control of gastrointestinal secretion.