To obtain evidence linking long-term potentiation (LTP) and memory, we examined whether LTP induction modifies functional relationship among neurons in the rat hippocampus. In contrast to neurons in low-frequency stimulated or AP5-treated slices, LTP induction altered "functional connectivity," as defined by the degree of synchronous firing, among simultaneously recorded neurons in the CA3 region. Interestingly, functional connectivity changed bidirectionally so that the total sum of functional connectivity remained constant. These results demonstrate LTP-induced changes in neuronal functional connectivity and suggest the existence of a normalization mechanism for the total sum of functional connectivity.