Rodents show pronounced local field potential (LFP) oscillations in the theta frequency range (7-12Hz in rodents) during exploration. Theta frequencies are thought to take part in memory processing by ensuring accurate sequences of neuron firing to facilitate plasticity processes. Many principal neurons in the parahippocampal areas, such as grid cells and place cells, show pronounced theta modulation. Grid cells in the medial entorhinal cortex (MEC) are theta modulated during active wakefulness, with power and frequency correlating to e.g. running speed or environmental familiarity. Suppressing endogenous theta oscillations by inactivating the medial septal area (MSA) leads to disintegration of grid cell firing fields, supporting the notion that theta activity is intimately associated with the grid cell’s role in navigation. However, recent experiments show that grid cells maintain their “gridness” when theta oscillations are optogenetically paced at frequencies outside of the endogenous theta range., pointing to a disconnection of theta activity and spatial representations. Here, we investigate the role of theta modulation on grid cell stabilisation in a novel environment. By expressing channelrhodopsins under a parvalbumin (PV) specific enhancer in the MSA in wild type rats, we optogenetically stimulated PV inhibitory neurons at 11Hz. LFP and single unit activity was recorded in the MEC while the animal explored a familiar and novel environment, whereupon the MSA was stimulated during initial exposures to the novel environment. This resulted in LFP and grid cell modulation locking onto the stimulation frequency. Ongoing analyses will reveal the effects on spatial representation in the MEC.