We have reported that porcine pancreastatin inhibits cholecystokinin (CCK)-stimulated pancreatic exocrine secretion in conscious rats. In the present study, the effects of mammalian pancreastatins on exocrine pancreatic functions in rats were compared using synthetic pancreastatins (porcine, bovine, human, and rat). Rats were prepared with cannulae draining pure pancreatic juice and bile separately and a duodenal cannula to return bile-pancreatic juice to the intestine and with a jugular vein cannula. After 90-min basal collection of pancreatic juice, CCK-8 (100 pmol/(kg.h] was infused for 3 h with or without pancreastatins (100 pmol/(kg.h]. All pancreastatins significantly inhibited protein output at an equivalent molar potency. These results suggest that mammalian pancreastatins have the same biological activity of a comparable magnitude and exert a similar biological action on the exocrine pancreas.