Neurons of the nucleus basalis-substantia innominata-nucleus of the ansa peduncularis complex (Ch4) provide the major source of cholinergic innervation for the entire neocortical surface. In contrast to their widespread projections to all parts of the neocortex, these neurons receive reciprocal projections from only very few cortical areas. Most of the sensory, motor, and association areas in the frontal, parietal, occipital and temporal lobes do not seem to project back to the Ch4 complex. The Ch4 neurons receive their cortical input from prepyriform cortex, orbitofrontal cortex, the anterior insula, the temporal pole, entorhinal cortex and the medial temporal cortex. There are also subcortical inputs from septal nuclei, the nucleus accumbens-ventral pallidum complex and the hypothalamus. This organization suggests that the Ch4 complex is in a position to act as a cholinergic relay for transmitting predominantly limbic and paralimbic information to the neocortical surface. It would also appear that the cortical areas which do not project into Ch4 have no direct way of controlling the cholinergic input which they receive, whereas the limited set of cortical areas which do project into Ch4 can control not only the cholinergic innervation that they receive but also the cholinergic innervation into the entire neocortical mantle.