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Theta is one of the most prominent extracellular synchronous oscillations in the mammalian brain. Hippocampal theta relies on an intact medial septum (MS) and has been consistently recorded during the training phase of some learning paradigms, suggesting that it may be implicated in hippocampus-dependent long-term memory processing. Object recognition memory (ORM) allows animals to identify familiar items and is essential for remembering facts and events. In rodents, long-term ORM formation requires a functional hippocampus but the involvement of the MS in this process remains controversial. We found that training adult male Wistar rats in a long-term ORM-inducing learning task involving exposure to two different, but behaviorally equivalent novel stimuli objects increased hippocampal theta power, and that suppressing theta via optogenetic MS inactivation caused amnesia. Importantly, the amnesia was specific to the object the animals were exploring when the MS was inactivated. Taken together, our results indicate that the MS is necessary for long-term ORM formation and suggest that hippocampal theta activity is causally linked to this process. [ABSTRACT FROM AUTHOR]