The coexistence of bovine pancreatic polypeptide (BPP) within the cell bodies and axons of noradrenergic neurons of the rat brain was studied. Adjacent hindbrain sections stained by the indirect immunofluorescence technique for either BPP or dopamine-beta-hydroxylase (DBH) revealed A1 and A2 cell bodies containing both antigens. Following bilateral knife cuts of the ventral noradrenergic bundle (VB), the localization of BPP, DBH and catecholamine within the VB and terminal fields of the hypothalamus was determined. An axoplasmic buildup of BPP immunofluorescent material was observed caudal to the knifecut together in the same axonal fields as catecholamine and DBH-containing axons. In the hypothalamus there was a marked decrease in the number of catecholamine-containing nerve fibers. However, there was no obvious decrease in the number of BPP fibers. The influence of high intraventricular doses of 6-hydroxydopamine on BPP and DBH-containing nerves in the hypothalamus was studied. While there was a marked reduction in DBH immunoreactive fibers, BPP fibers appeared relatively unchanged. These results suggest that BPP coexists within some hindbrain catecholamine neurons and their axons. It is suggested that BPP and catecholamine coexist in terminal fields within the hypothalamus. Failure to reveal a decrease in the BPP-fibers in these fields suggests that sprouting of arcuate nucleus derived non-aminergic BPP-containing fibers has occurred.