There is overlap between the structures containing head direction (HD) cells and those mediating the hippocampal theta rhythm, and both signals are thought to play an important role in spatial navigation. Previous research has shown that reversible inactivation of the medial septum attenuates hippocampal theta activity and disrupts path integration-based navigation. Although the HD signal reflects navigational performance, it is unclear whether theta rhythm contributes to the direction-specific activity of HD cells. We sought to determine whether HD cell activity is changed following reversible inactivation of the medial septum to eliminate theta rhythm. HD cells were recorded from the anterodorsal thalamus of female Long-Evans rats while they navigated in a cylindrical environment across standard, landmark rotation, and dark conditions. Following infusions of muscimol into the medial septum, recordings demonstrated a clear decrease in the power of hippocampal theta oscillations. In the landmark rotation experiment, intraseptal administration of muscimol produced HD cells with preferred directions that shifted unpredictably between sessions, suggesting that cue control was affected. Further, following septal inactivation many HD cells were unable to maintain a stable preferred firing direction within the recording sessions when the animals locomoted in the dark, suggesting that idiothetic processing was affected. These findings provide evidence that theta oscillations contribute to the directional stability of HD cells in anterodorsal thalamus. (PsycInfo Database Record (c) 2021 APA, all rights reserved).