The hippocampus is widely believed to play an important role in learning and memory. Long-term potentiation is currently viewed as a model for memory storage in the nervous system. The overall goal of this dissertation proposal is to characterize the excitatory post synaptic potential (EPSP) and induction parameters of long-term potentiation (LTP) at the perforant path to CA1 synapse of the rat hippocampus, in vivo. These experiments will complete preliminary studies demonstrating the methods for identifying perforant path-CA1 EPSPs, in vivo. Preliminary data has also shown that the gamma-aminobutyric acid (GABA) agonist baclofen, a selective GABAB receptor agonist, produces reliable and robust LTP in this pathway. Therefore, the role of GABA13 receptors will be investigated at this synapse. In addition, immunohistochemical data show selective opiate staining is present in the lateral aspect of the perforant path synapse of area CA1. Therefore, these studies will investigate the role of opiates and their antagonists, as well as the involvement of the NMDA receptor in perforant path-CA1 LTP. Experiments will be conducted on male rats during acute stereotaxic surgery, and field potentials recorded extracellularly.