The exocrine pancreas secretes throughout 24 hours. In both interdigestive and postprandial states, pancreatic secretion is regulated by neural and hormonal actions, and neurohormonal interactions. At the turn of this century, the prevailing theory of Pavlov (1) that pancreatic exocrine secretion was exclusively regulated by secretory fibers in both vagi and splanchnic nerves was refuted by Bayliss and Starling in 1902, who put forward their hypothesis that pancreatic secretion was stimulated by a circulating hormone, "secretin," which was released by hydrochloric acid from the duodenal mucosa (2). Soon, Pavlov conceded to the hypothesis of Bayliss and Starling. Pavlov adopted the theory of a dual mechanism--nervous and hormonal--of the regulation of pancreatic secretion. In recent decades, explosions of new information about old and newly discovered gut hormones or peptides and neuropeptides have resulted in a better understanding of the regulatory mechanism of the exocrine pancreas, and also opened new, exciting frontiers in the investigations of the physiology and pathophysiology of pancreatic secretion.