Secretin is a gut peptide hormone that is also expressed in the CNS. To explore the potential neuroactive role of secretin in the brain, we have generated secretin deficient mice. Secretin deficient mice demonstrated impairment in synaptic plasticity (significant reduction in long term potentiation (LTP) induction and LTP maintenance) in the CA1 area of the hippocampus. Using a beta-galactosidase (lacZ) reporter in the targeted allele and secretin antibody staining, we have detected secretin gene expression in the hippocampus, cerebellum, and the brain stem in adult mouse brain. In the hippocampus, secretin was expressed in the dentate gyrus, the hilus, and the molecular layer. These findings suggest that secretin is involved in synaptic function in the adult brain.