Abstract The intra-hippocampal organization of theta rhythm was examined in chronically implanted rats. Small lesions were placed in the posterior aspects of the hippocampal formation and the subsequent effect of the lesion ascertained electrophysiologically. Lesions of the subiculum significantly diminished the amount of theta rhythm present and the effect appeared to be permanent. Lesions of the lateral entorhinal region produced a large, but transient, increase in the amount of theta rhythm present. The electrophysiological consequences of these lesions were organized in ‘lamellae’ as previously suggested. In addition, the effects of the discrete lesions were expressed ipsilaterally only. The fiber connections underlying these effects were reviewed and the subiculum was postulated to be a critical region for the maintenance of normal ‘on-going’ hippocampal electrical activity. The increase in theta rhythm following lateral entorhinal lesions was postulated to be result of the loss of high-frequency influences from other neural regions. It was concluded that, although paced by the septal regions, hippocampal theta rhythm is dependent upon intra-hippocampal formation maintenance and modulation. In particular, the present data indicate the importance of the subiculum in this maintenance. Furthermore, the data suggest an important role of the hippocampal formation in the organization of theta rhythm.