The hippocampus is one of the more widely studied structures related with spatial memory. In this study, we assessed the effect of unilateral inactivation of the dorsal hippocampus with tetrodotoxin (TTX) on the performance displayed by Wistar rats in the spatial version of the Morris water maze. In experiment 1, we injected into the dorsal hippocampus in two different groups of rats 1 microl of saline solution or 5 ng of TTX in 1 microl of saline each day immediately after the training during four consecutive days. This procedure blocked consolidation and impaired spatial memory in the TTX group. In experiment 2, a new group of subjects was trained in the Morris water maze for 8 days and was administered 1 microl of saline on day 7 (saline session) and TTX on day 8 (TTX session) into the dorsal hippocampus 40 min before the training. Only the treatment with TTX altered the retrieval of memories. These experiments showed that unilateral interventions on the dorsal hippocampus can affect consolidation as well as retrieval of well-established spatial memories.