The morphological organization of the two principal efferent pathways of the hippocampal formation - the fimbria and the dorsal fornix - and of the interhippocampal fibers that cross in the ventral hippocampal commissure, has been studied autoradiographically following injections of 3H-amino acids into the relevant cytoarchitectonic fields. To a considerable extent the rostrally-directed fibers from each cytoarchitectonic field remain topographically segregated within the fimbria or dorsal fornix, their locations being determined by the relative levels at which they join these bundles. Thus, the fibers from the subicular region tend to be aggregated along the ependymal border of the fimbria while those from the hilar region of the dentate gyrus are medially-placed along its pial margin. Similarly, the efferents from the septal and temporal parts of the various fields are distributed along the proximo-distal dimension of the fimbria, from its base (adjacent to field CA3) to its tip. The dorsal fornix contains fibers from the septal (dorsal) part of the subiculum and field CA1; although there appears to be some overlap, the subicular efferents tend to be more medially placed while those from CA1 are further lateral. The topographic organization of the fibers in the fimbria is reflected in the arrangement of the crossed components in the hippocampal commissure. Those arising at more temporal levels cross most rostrally while those from the septal pole of field CA3 cross in the caudal part of the commissure; those from the hilar region of the dentate gyrus cross most ventrally, while those from the more lateral parts of field CA3 (CA3b and CA3a) cross at successively more dorsal levels. We estimate that the fimbria contains about 900,000 fibers of which approximately 700,000 are myelinated (with internal diameters ranging between 0.1 and 2.8μm; mean 0.78 μm) and just over 200,000 are unmyelinated (with diameters between 0.1 and 0.8 μm; mean 0.24 μm). © 1980 Springer-Verlag.