Diagrams of the lymph node currently represent its deep cortex (paracortex) as a layer of rather uniform thickness underlying the whole peripheral cortex. However, this concept has not been supported by actual observations; previous nvestigators have observed, instead, related structures whose appearance varied greatly from nodule-like to ill-defined components. Clearly, the present knowledge of the histology of the deep cortex is inadequate and confusing. Therefore, we undertook a tridimensional study of the region in different nodes of rats. The present work, bearing on the topography of the region, revealed that the deep cortex of the rat node is formed of one to several basic “units”. Each unit is a semi-rounded structure, varying from semispheric to semi-ovoid in shape and contiguous to a portion of peripheral cortex. The work further showed that two to several units can fuse to form a “complex”. The data indicated that the number, the size and the shape of the units and/or of the complexes of a node differ to some extent according to its anatomical location. These differences probably reflect corresponding variations in the nature and importance of the antigenic stimulation in the different sites of the organism. Finally, the study demonstrated the necessity of tridimensional examination of a node to obtain adequate information on its overall architecture and, particularly, on its deep cortex topography.