1. Results obtained with the anterograde horseradish peroxidase (HRP) and autoradiographic techniques have revealed a dense and compact spinal and trigeminal projection to the dorsal portion of the nucleus submedius (Sm) in the medial thalamus of the cat. A homologous projection was observed in two cases in the monkey and in three cases in the rat with the anterograde HRP method. 2. The projection to Sm in topographically organized rostrocaudally, with trigeminal terminations located caudally and spinal terminations rostrally. The rostrocaudal extent of the trigeminal field in Sm (ca. 1 mm) is larger than that of the spinal field (ca. 0.8 mm). There is a considerable ipsilateral trigeminal projection, whereas there is a minimal uncrossed spinal projection to Sm. 3. Retrogradely labeled cells were analyzed in cases with small injections of HRP in medial thalamus in the cat. The results indicate that the projections to Sm identified in the anterograde experiments arise from lamina I cells of the spinal and medullary dorsal horm, probably exclusive of lamina V and other spinothalamic cells. The retrograde results corroborate the topographic observations made in the anterograde experiments. 4. These results suggest that Sm may be critically involved in specific nociception.