The metabotropic glutamate receptor, mGlu5, plays a critical role in the persistency of hippocampal long-term potentiation (LTP), long-term depression (LTD) as well as hippocampus-dependent associative learning and memory (doi: 10.3390/cells11213352). Although well-studied, the majority of investigations as to the role of mGlu5 in these processes in rodents has been done during the light cycle that amounts to the resting phase of rodent species. Sleep-states are regulated by mGlu5 (doi: 10.3390/cells12131761) suggesting that its contribution to hippocampal information processing and storage may differ across the diurnal cycle.Here, we recorded field potentials from Schaffer collateral (SC)-CA1 synapses of the dorsal hippocampus of freely behaving transgenic (mGlu5-/-) mice and their wildtype(wt) littermates in the daytime and at night (events separated temporally by 12 h) to assess whether the contributions of mGlu5 receptors to LTP vary across the light-dark cycle.Hippocampal LTP that was induced by high frequency stimulation of SC inputs to CA1 was significantly impaired in mGlu5-/- mice (compared to wt controls) regardless of the phases of the diurnal cycle. Differences in the profile of plasticity changes and in responses to metaplastic priming suggested nonetheless that diurnal variations in the regulation by mGlu5 receptors of LTP may occur.Supported by the Deutsche Forschungsgemeinschaft (SFB 1280/A04, project number: 316803389).