The effects of a synthetic putative luminal cholecystokinin (CCK) releasing factor (LCRF) fragment (1–35) on pancreatic exocrine secretion were examined in conscious Wistar rats and a mutant strain of rats lacking the CCK-A receptor. Intraduodenal injection of graded doses of the LCRF fragment induced biphasic responses in Wistar rats. The injection of 0.1 μg of the LCRF fragment produced a maximal response, while 1 μg produced a lower response. No significant effect was observed in rats lacking the CCK-A receptor. The synthetic LCRF fragment stimulated pancreatic secretion via CCK-A receptors in conscious rats.