The origin and termination of fibers to the mediodorsal thalamic nucleus, especially those to the medial, magnocellular part of the nucleus (MDm), have been studied using anterograde and retrograde axonal tracing methods, as well as electrophysiological recording. The results indicate that in addition to its well-known connections to and from the prefrontal cortex, MDm receives fibers from many parts of the basal forebrain, including the ventral pallidum and other parts of the substantia innominata, the amygdaloid complex, the primary olfactory cortex, entorhinal and perirhinal cortex, and the cortex at the pole of the temporal lobe. Lighter projections arise in the subiculum, the ventral insula, and the superior and inferior temporal gyri. The cells that project to MDm tend to be large, polymorphic neurons. Throughout most of the basal forebrain they are diffusely distributed through several nuclei or cortical layers, without obvious relation to nuclear or laminar boundaries. The major exception to this is in the ventral pallidum, where there is a dense concentration of cells that project to MDm. The lateral part of the mediodorsal nucleus (MDl) receives few if any fibers from the basal forebrain and temporal lobe, but is innervated by several brainstem structures, especially the superior colliculus, the substantia nigra, the medial vestibular nucleus, and the midbrain tegmental fields. In MDm, the fibers are distributed in irregular patches. Three-dimensional analysis indicates that these patches are often clustered into separate bands or columns at different anteroposterior levels. In addition, the strongest projections from the three major regions that innervate MDm are organized in a complex three-dimensional pattern. First, the fibers from the amygdaloid nuclei terminate most heavily (but not exclusively) in the rostral third of MDm. The parvicellular accessory basal amygdaloid nucleus and the amygdalohippocampal area project principally to the dorsal part of the nucleus. The parvicellular basal nucleus and the periamygdaloid cortex project to the ventromedial quadrant of MDm; and the magnocellular basal nucleus, the magnocellular accessory basal nucleus, and the lateral nucleus all project to the ventrolateral quadrant. Second, the substantia innominata projects preferentially to the caudal part of MDm. The medial part of the substantia innominata, especially the ventral pallidum, innervates the dorsomedial quadrant, while more caudal and lateral areas of this region project ventrolaterally. Third, the projections arising from the entorhinal and other temporal cortical areas terminate primarily in the mid-rostrocaudal level of MDm.(ABSTRACT TRUNCATED AT 400 WORDS)