Recent studies in genetically obese and diabetic Otsuka Long-Evans Tokushima Fatty (OLETF) rats suggest defects of cholecystokinin (CCK)-A receptor gene expression and CCK-A receptor-mediated biological functions such as pancreatic juice, protein, and gastric acid secretion. The present studies were undertaken to further examine CCK-A receptor gene expression and CCK-A receptor-mediated biological functions in the pancreas, stomach, and brain of OLETF rats. Expression of the CCK-A receptor gene could not be detected in the stomach, pancreas and brain by the reverse-transcription polymerase chain reaction (RT-PCR) method and Southern blotting of the PCR products. Southern blot analysis of genomic DNA from OLETF and control Long-Evans Tokushima Otsuka (LETO) rats with CCK-A receptor fragment as a probe revealed different restriction bands. Expression of the CCK-B receptor gene was observed in the stomach, pancreas, and brain in both OLETF and LETO rats by the RT-PCR method, with expression of the CCK-B receptor gene markedly enhanced in OLETF rats compared with that in LETO rats. Consistent with the defect of CCK-A receptor gene expression, CCK-A receptor-mediated biological functions were not observed in these organs. Perfused exocrine and endocrine pancreas of OLETF rats were insensitive to CCK stimulation but not to carbamylcholine stimulation. Basal gastric acid and pepsinogen secretions in OLETF rats were higher than in LETO rats. OLETF rats showed a significantly higher average daily food intake, gained body weight faster, and were heavier than LETO rats. The present study confirmed that OLETF rats have CCK-A receptor gene anomalies and demonstrated deficient CCK-A receptor-mediated biological function in the pancreas, stomach, and brain.