The afferent and efferent connections of the dorsal tegmental nucleus (DTN) were studied in the rat using axoplasmic transport techniques. Horseradish peroxidase (HRP) and Fast Blue were injected stereotaxically into either pars centralis or pars ventromedialis of the DTN, two subdivisions of the nucleus with distinctive connections. The pars centralis is reciprocally connected with the ipsilateral lateral mammillary and interpeduncular nuclei; these projections constitute the major afferent and efferent systems of the DTN. Commissural fibers from the corresponding pars centralis and intrinsic fibers systems are massive and form a complex fiber meshwork within the subnucleus. The prepositus hypoglossi nucleus (ipsilateral) and supragenualis nuclei (bilateral) also project to the pars centralis. Smaller numbers of afferent fibers arise from the lateral habenular nucleus, the posterior hypothalamus and the brainstem reticular formation. The pars ventromedialis of the DTN receives diverse inputs which include the septal nuclei, diagonal band of Broca, preoptic area, anterior and lateral hypothalamus, lateral and medial habenular nuclei, medial mammillary nucleus and many nuclei of the brainstem reticular formation. Based on the differences of connections and cytoarchitecture between the pars centralis and the pars ventromedialis, the pars ventromedialis may be an entity separate from the dorsal tegmental nucleus.