The neuronal types in the ventral nucleus of the cat medial geniculate body projecting to the primary auditory cortex (AI) were investigated using the retrograde transport of horseradish peroxidase. These cells were compared with the morphology of neurons as revealed in Golgi and Nissl preparations, plastic-embedded tissue, and electron microscopic material. After large injections, more than 90% of the neurons in the ventral nucleus, the principal nucleus of the lemniscal auditory pathway, were labeled, and the population of labeled cells included both large and small neuronal somata. Since the ventral nucleus contains only two varieties of cells--large neurons with bushy dendrites and an unbranched axon, and smaller cells with thin dendrites and a locally projecting axon--it is concluded that at least some of the small cells, previously believed to be interneurons, may function both as local circuit and as projection neurons. These findings were confirmed in toluidine blue-stained, 1-2 micron thick sections, and in the electron microscope, where small cells with sparse cytoplasm and a deeply invaginated nuclear envelope often contained intracellular horseradish peroxidase granules, as well as the larger neurons. Besides the small, labeled neurons in the ventral nucleus, many labeled cells were seen in the interstitial nucleus of the brachium of the inferior colliculus. This hitherto poorly characterized group of cells is embedded among the fibers of the brachium of the inferior colliculus. Many of the morphologically distinct varieties of cells in the medial division of the medial geniculate body, including small neurons, were labeled. Thus, in addition to the route embodied by the large bushy neurons which project to primary auditory cortex, at least one other pathway--represented by certain of the small cells in the ventral nucleus, reaches the primary auditory cortex.