Spatial memory formation is a dynamic process requiring a series of cellular and molecular steps, such as gene expression and protein translation, leading to morphological changes that have been envisaged as the structural bases for the engram. Despite the role suggested for medial temporal lobe plasticity in spatial memory, recent behavioral observations implicate specific components of the striatal complex in spatial information processing. However, the potential occurrence of neural plasticity within this structure after spatial learning has never been investigated. In this study we demonstrate that blockade of cAMP response element binding protein-induced transcription or inhibition of protein synthesis or extracellular proteolytic activity in the ventral striatum impairs longterm spatial memory. These findings demonstrate that, in the ventral striatum, similarly to what happens in the hippocampus, several key molecular events crucial for the expression of neural plasticity are required in the early stages of spatial memory formation. T he formation of long-term memories is believed to involve a dynamic process by which a labile memory is progressively converted into a more stable and potentially permanent trace. Evidence for such a time-dependent process comes from studies demonstrating that electroconvulsive shock produces amnesia only if delivered shortly after learning, whereas the same treatment is ineffective when delivered several hours later (1). Long-term memory is accompanied by changes in neuronal morphology and connectivity, and these alterations are thought to be essential for the stabile encoding of new information (2). This transformation has been suggested to depend upon plastic changes that involve a sequence of specific and coordinated cellular processes. These begin with neurotransmitter receptor activation that induces shortterm changes in synaptic efficacy based on receptor phosphorylation and trafficking (3, 4). Subsequently, alterations in gene expression and protein synthesis occur that are the basis for longterm structural modifications (5, 6).A key issue in the study of memory in vertebrates is the brain site at which these processes occur. Clinical evidence in humans suggests that structures within the medial temporal lobe (MTL) play a prominent role in long-term memories. MTL lesions induce profound deficits in the formation of long-lasting declarative memories while sparing the acquisition of nondeclarative memories such as visual-motor skills (7,8). Such findings suggest that the hippocampus might be an essential site where plasticity occurs in the initial steps of declarative memory stabilization. Accordingly, those molecular events thought to be crucial for the long-term encoding of memories such as regulation of gene expression and protein synthesis, as well as structural changes, have been described in the hippocampus after spatial learning (6, 9, 10).Recent experimental evidence, however, demonstrates that structures different from the hippocampus might also be involved in ...