An Otsuka Long-Evans Tokushima Fatty (OLETF) rat does not possess cholecystokinin (CKK)-A receptor gene expression because of a genetic abnormality. We examined the CCK release and its transcription in OLETF rats in comparison with control (Long-Evans-Tokushima-LETO) rats. Animals at 5-6 and 24-25 weeks of age were used. The level of CCK mRNA and CCK concentration in the small intestinal mucosa, and the level of CCK-A receptor mRNA in the pancreas were examined. To examine CCK release, rats were prepared with external bile and pancreatic fistulae. The basal plasma levels of CCK and those at 2 h after bile-pancreatic juice diversion were measured by radioimmunoassay. The levels of CCK mRNA, and tissue CCK concentration increased significantly with age in both strains, but the values in OLETF rats were significantly lower than those in LETO rats. The CCK-A receptor mRNA level also increased with age in LETO rats. The plasma CCK concentration was significantly increased by bile-pancreatic juice diversion regardless of age and strain; however, the level of plasma CCK in OLETF rats at 24-25 weeks of age was significantly lower than that of LETO rats. It is suggested that the long-term defect of CCK-A receptor may decrease CCK release and transcription.