This study describes the organization of cells in the thalamic reticular nucleus (TRN) that project to the auditory part of the cat's dorsal thalamus. Injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) and fluorescent dyes were made into the medial geniculate complex (MG). The resultant retrograde labelling in the TRN was analyzed. Injections of WGA-HRP into the ventral (MGv), dorsal (MGd), or medial (MGm) nuclei of the MG label zones of cells that are restricted to a caudoventral sector of the TRN. In reconstructions, these zones resemble "slabs" that are elongated in the dorsoventral and oblique rostrocaudal dimensions of the nucleus. In comparisons of the zones of labelling in the TRN following tracer injections into different nuclei of the MG, inner and caudal cells project to the pars lateralis of the MGv (MGvl) or to the MGd, and outer and rostral cells project to the pars ovoidea of the MGv (MGvo) or to the MGm. Thus, cells projecting to the MGvl or MGd or to the MGvo or MGm occupy overlapping territories. Double injections of different fluorescent dyes into selected pairs of MG nuclei result in reticular cells that are labelled from either both nuclei or only one or the other nucleus in each pair. These results indicate that the projections of cells in the auditory sector of the TRN to the MGvl or MGvo or to the MGd or MGm are topographically organized. Furthermore, projections to more than one MG nucleus can arise from single reticular cells.