Abstract In this investigation the projections of the hippocampal formation to the septal area and hypothalamus were studied in the rat with the combined use of 3H-amino acid radioautography and horseradish peroxidase histochemistry. The results indicate that all of the fibers which project to the hypothalamus and the majority of fibers which project to the septum arise from the subicular cortex and not from hippocampal pyramidal cells. The projection to both of these areas are topographically organized along the longitudinal axis of the hippocaapal formation. Specifically, fibers from subicular cortical cells situated at the septal end of the hippocampal formation which project through the medial part of the dorsal fornix terminate in the dorsomedial quadrant of the lateral septal nucleus and in the dorsal portion of the pars posterior of the medial mammillary nucleus. Fibers from progressively more posteroventral levels of the hippocampal formation which project through more lateral portions of the dorsal fornix and fimbria terminate in progressively more lateral and ventral quadrants of the lateral septal nucleus and in progressively more ventral portions of the pars posterior. Concerning the specific origin of the fornix system, fibers from only the prosubiculum and subiculum project through both the pre- and postcommissural fornix. Hippocampal pyramidal cells from all CA fields have a restricted projection through the precommissural fornix and terminate in the caudal half of the septum while the presubiculum projects solely through the postcommissural fornix.