Septal inputs critically modulate hippocampal function. However, the role of Medial Septal/Diagonal Band (MS/DB) neuronal populations in hippocampal theta rhythm generation is still under debate. MS/DB GABAergic neurons selectively innervate inhibitory hippocampal interneurons and through this action exert a powerful control on hippocampal excitability. To further investigate the role of the MS/DB GABAergic neurons in hippocampal theta rhythm generation, the antibody Gamma amino butyric acid Transporter 1 (GAT-1) conjugated to saporin (SAP, 3µL at 325ng/µL) was injected in the MS/DB of adults Sprague Dawley rats to selectively destroy GABAergic neurons. Population estimates of MS/DB GABAergic neurons were analyzed using StereoInvestigator 9. A significant reduction (75%, p<0.0001) in the MS/DB GABAergic population was observed, while the cholinergic and glutamatergic neuronal population were spared. The effects of this GABAergic neuronal loss in hippocampal function were investigated by hippocampal EEG recordings. EEG recordings of freely moving rats were acquired using the Harmonie Stellate monitoring system. EEGs were recorded before and after the intraseptal injection of anti-GAT-1-SAP. Two second duration trace, representative of theta I and theta II were transferred off line to ClampFit 9 software to measure amplitude and produce a power spectrum. After analysis of the theta traces the rat brains were sliced and stereology was once again performed to determine the relationship between the destruction of MS/DB GABAergic neurons and the hippocampal theta rhythm.