We investigated the effects of somatostatin (SMS)-201-995, atropine, and MK-329 on the role of cholinergic- and cholecystokinin-related systems and on the secretory relationship between five pancreatic digestive enzymes in rats. Animals kept in restraint cages and provided with pancreatic, biliary, duodenal, and jugular vein cannulas were treated as follows: 1) 0.25 micrograms.kg-1.h-1 caerulein alone, 2) both 0.25 micrograms.kg-1.h-1 caerulein and 100 micrograms.kg-1.h-1 atropine, 3) both caerulein and 5 micrograms.kg-1.h-1 SMS, 4) 91.3 micrograms.kg-1.h-1 carbachol alone, 5) both carbachol and 0.5 mg.kg-1.h-1 MK-329, and 6) both carbachol and 5 micrograms.kg-1.h-1 SMS, respectively. Food, but not water, was denied rats starting 10 h before the experiment and throughout the 6-h experimental period. The secretory patterns over the 6-h experimental period showed noticeably independent regulation of pancreatic secretion of individual digestive enzymes. The relationship between paired enzymes significantly varied according to the treatment. The correlation between chymotrypsinogen and the other enzymes was markedly modulated by MK-329. Our results suggest that SMS is a major "gate-keeper" in the regulation of exocrine pancreatic secretion and that the secretion of each digestive enzyme is individually regulated. Furthermore, they suggest that cholecystokinin and acetylcholine and their respective agonists are essentially initiators of secretory processes of the pancreas. Therefore, the paradigms of the regulation of pancreatic secretion heretofore accepted should be reexamined.