Using the powerful method of DNA amplification, we have been able to isolate several cholecystokinin B (CCKB) receptor (CCKB-R) mRNA forms from rat brain tissue, allowing the detection of a truncated mRNA species and the determination of the CCKB-R gene structure. Unspliced precursor mRNA and the mature form were identified in the cerebral cortex, hypothalamus, and hippocampus in apparently differing proportions according to the region examined, suggesting that the expression of the CCKB-R could be modulated at a posttranscriptional level. In the case of the cerebellum, only a completely unspliced mRNA form was detected, in agreement with previous studies in which CCKB ligand binding sites were not observed. In contrast, a truncated CCKB-R mRNA, lacking 250 bp, was detected in all the studied brain regions except for the cerebellum. This mRNA, for which a cellular function has not been assigned, potentially encodes a protein consisting of 168 amino acids.