The distribution of neurons in diencephalon and brainstem which project upon the hippocampal formation has been analyzed in adult rats by the injection of horseradish peroxidase into different parts of the hippocampus and dentate gyrus and the related retrohippocampal structures, including the subicular complex and the entorhinal cortex. Any large injection of horseradish peroxidase inlo the hippocampal region results in the retrograde labeling of some neurons in each of the following structures: in the thalamus-the nucleus reuniens, the parataenial and paraventricular nuclei, the anterodorsal and antermedial nuclei, and the laterodorsal and lateral posterior nuclei; in the hypothalamus, septum and preoptic region-the medial septal nucleus and the diagonal band of Broca, the substantia innominata, the lateral preoptic area, the magnocellular preoptic nucleus, and the anterior amygdaloid area, the dorsomedial hypothalamic nucleus, the lateral and posterior hypothalamic areas, the ventral premammillary nucleus, the supramammillary region, and parts of the tuberomammillary and lateral and medial mammillary nuclei: in the brainstem: the ventral tegmental area, the substantia nigra, the interpeduncular and interfascicular nuclei, the dorsal and median nuclei of the raphe, the dorsal and laterodorsal tegmental nuclei, the locus coeruleus, the central gray, and certain of the tegmental reticular fields. It is clear that these afferents to the hippocampal formation do not comprise a single, homogeneous system, and that their terminations within the hippocampal region are not restricted to a particular topographic level (i.e. septal, intermediate or temporal). © 1979.