The ventral posteromedial nucleus (VPM) of the monkey thalamus was investigated with correlative anatomical and physiological techniques. On the basis of staining for cytochrome oxidase (CO), VPM is divided into a lightly stained, background matrix domain and an intensely stained rod domain. The latter consists of elongated rods of large, medium, and small cells, 500 microns wide on average and extending anteroposteriorly, many of them through the full extent of the nucleus. The matrix, consisting of small cells, penetrates between the rods and expands at the dorsomedial, ventrolateral, and posterior aspects of VPM. Multiunit mapping reveals that VPM contains a dorsally situated representation of the contralateral side of the head, face, eye, and interior of the mouth and a medially situated representation of the ipsilateral side of the lips and interior of the mouth, and that the same small region is represented in the same relative position through the full anteroposterior extent of the nucleus. Earlier work had shown that single CO rods contain the representation of the same portion of the periphery throughout their length. The present study suggests that rods in equivalent positions may represent the same portion of the periphery from animal to animal. The cells of the rod and matrix domains show different patterns of immunoreactivity. Virtually all of the large- and medium-sized rod cells are immunoreactive for the calcium-binding protein parvalbumin, and many are stained by the monoclonal antibody CAT 301. Small GABA-immunoreactive cells and terminal-like puncta are highly concentrated in the rods but are dispersed in the matrix. In the matrix, all non-GABA cells are small, immunoreactive for 28-kDa calbindin, and not stained by CAT 301. They appear to form part of a wider system of calbindin-positive cells that extends into adjacent nuclei. The CO rods are indicative of the modularity of the lemniscal component of the trigeminal part of the somatic sensory system at thalamic levels. Thalamocortical relay neurons in this compartment of VPM express a calcium-binding protein and a surface proteoglycan that distinguishes them from relay neurons in the matrix compartment of the nucleus. In the following paper (Rausell and Jones, 1991), the rod and matrix compartments are shown also to have different patterns of input and output connections.