Long-term potentiation (LTP), a cellular model for long-term memory, is generally acknowledged to consist of both a short-term phase that is characterized by a dependence on autonomous protein kinase activity, and a long-term phase that is characterized by a dependence on changes in gene expression and new protein synthesis. Similarly, long-term memory exhibits a dependence on gene expression and altered protein synthesis. Recent evidence indicates that the mitogen-activated protein kinase (MAPK) cascade plays a role in both LTP and long-term memory. The MAPK cascade has heretofore largely been studied in the context of cell division and proliferation and as such, mechanisms for the regulation of gene expression by the MAPK cascade have received considerable attention. Given the possible role of altered gene expression in the late phase of LTP and in long-term memory, we evaluated the capacity of the MAPK ERK (extracellular signal-regulated kinase) to regulate phosphorylation of the transcription factor cAMP response element binding protein (CREB) in hippocampal area CA1. Our studies indicate a critical role for the MAPK cascade in the regulation of CREB phosphorylation in the hippocampus.