Morphological changes, including changes in size, shape, and number of synapses, in neurons have been observed in many species and are thought to be critical for long-term memory storage. Actin filaments are intimately involved in neuronal morphology and regulation of their dynamics can influence memory. Rho GTPase plays a prominent role in this process and has been implicated in both pre- and post-synaptic morphological changes. Therefore, we examined the effect of hippocampal manipulation of Rho and ROCK activity on performance in a spatial memory task. Post-training intrahippocampal infusion of an inhibitor of the downstream effector kinase p160ROCK impaired long-term memory. Furthermore, post-training activation of Rho using lysophosphatidic acid (LPA) enhanced long-term spatial memory. This memory enhancing effect of LPA was not mediated via the Erk cascade, as no change in Erk phosphorylation was observed as a result of its administration. Our results demonstrate a role for the Rho-ROCK pathway in hippocampus-dependent spatial memory. (C) 2004 Elsevier Inc. All rights reserved.