In summary, we have demonstrated the subnuclear organization of PB, and correlated this with the origins of its efferent connections. In general, PBm projects primarily to the insular, infralimbic and lateral frontal cortex, and to associated areas in the thalamus, hypothalamus and amygdala. PBl chiefly innervates the autonomic nuclei of the hypothalamus and related portions of the amygdala and the bed nucleus of the stria terminalis. KF is the main source of descending projections from PB to the region of the nucleus of the solitary tract, the ventrolateral medulla and the intermediolateral cell column in the thoracic spinal cord. Further subnuclear organization of the origins of these projections within the major PB subdivisions has been described in detail. While PB afferents tend to terminate in specific subnuclei, one cannot reliably predict from the functional properties of the major inputs to a subnucleus what information will be carried in its efferents. Further anatomical and physiological studies of the input-output relationships of single PB neurons will be necessary to help resolve this enigma. However, recent immunohistochemical observations suggest that the subnuclear organization of PB afferent and efferent connections may reflect, at least in part, their biochemical specificity.