To determine the effect of long-term blockade of cholecystokinin (CCK) on both pancreatic storage and secretion processes, L 364,718 (a CCK receptor antagonist) was administered to rats at 0.1 mg/kg/day for 3, 7, and 15 days. Zymogen granules were analyzed by flow cytometry to determine their light scattering properties—forward scatter and side scatter—as well as their amylase content measured by a specific antiserum. The mean number of zymogen granules per cell was counted on pancreatic sections using electron microscopy. DNA content, pancreatic weight, and enzyme secretion were also studied under both basal conditions and CCK infusion at a dose of 1.25 μg/kg/h, which is able to displace the CCK receptor antagonist. Two subpopulations of zymogen granules (Z1and Z2) were identified on the basis of their light scattering parameters, in both control and L 364,718-treated rats. L 364,718 administered for 3, 7, and 15 days induced a significant reduction in the amylase content of individual zymogen granules, for both Z1and Z2zymogen granule subsets. In contrast, the number of zymogen granules per cell increased from day +3 of treatment onward, the highest values being detected at day +7. Hyperplasia was observed only at day +15. Basal enzyme secretion decreased significantly in rats treated with L 364,718 for 3 and 7 days but recovered to control values after 15 days of treatment. No significant differences in CCK- stimulated amylase secretion were observed between control and L 364,718-treated rats. At day +15 of L 364,718 treatment a significant increase in enzyme secretion was observed with respect to shorter treatment periods; this was associated with a significant increase in both the number of cells and the number of zymogen granules per cell. Our results indicate that chronic administration of L 364,718 induces a biphasic effect on pancreatic function. Interestingly, although enzyme secretion reached recovery after long-term treatment (15 days), the storage process is altered since the mean enzyme content in each individual zymogen granule remains significantly reduced.