Terminal labelling within the parabrachial nucleus arose as a caudal and lateral extension of the fibers coursing through the periaqueductal gray matter and central tegmental fields (see Fig. 7B; see also Fig. 3L,M). In the rostral parabrachial nucleus, a few labelled fibers of fine caliber found in the superior lateral subnucleus exhibited infrequent swellings along their lengths. Within the central lateral subnucleus there was a denser collection of spinelike processes and boutons along the lengths of fine caliber fibers as they coursed laterally from the periaqueductal gray matter. Both medium and fine caliber fibers, observed within the dorsal and external lateral subnuclei, exhibited boutons and varicosities along their lengths. In the ventrolateral parabrachial subnucleus fine caliber fibers were observed, which also demonstrated varicosities along their lengths. In caudal sections through the parabrachial nucleus, labeling within these subnuclei grew denser. Labeled fibers within the medial subnucleus were oriented in a mediolateral direction. These tended to be smooth with occasional swellings along their lengths in the more lateral part of the subnucleus. In the more medial part of the nucleus, these labelled fibers branched and gave rise to finer axons with complex bouton-bearing processes. Collator note: Fig 3L-M roughly corresponds to Swanson Atlas Levels 47-51. The terminals fields in PB are organized around the superior cerebellar peduncle. KF is not labeled in this experiment.