The projections of brainstem catecholaminergic (CA) cell groups to the rat nucleus tractus solitarius (NTS) were examined using 6-hydroxydopamine (6-OHDA) injections and glass microknife cuts. 6-OHDA (4 micrograms) was injected into the intermediate NTS, and this resulted in depletion of CA fluorescent varicosities from the NTS at this rostrocaudal level, except for varicosities along the periventricular edge of the NTS. In addition, a band of swollen fluorescent axons extended between the CA A1 cell group of the ventrolateral medulla and the lateral NTS. Microknife cuts were used to interrupt the projections of the CA A1 and A2 cell groups (located in the caudal NTS) and tissues were examined for changes in CA varicosity density within the intermediate NTS. Following transverse knife cuts of the intermediate NTS, rostral to the A2 cell group, fluorescent varicosities rostral to the cut virtually disappeared, and the fluorescence intensity of the ipsilateral A2 neurons caudal to the cut was increased. These cuts also eliminated the 6-OHDA-resistant varicosities along the periventricular NTS. After microknife cuts lateral to the intermediate NTS, the fluorescent varicosity density in the NTS was unchanged. These results indicate that the major CA projection to the NTS arises from the ipsilateral A2 cell group. The 6-OHDA-resistant varicosities arising from neurons caudal to the knife cut probably arise from the adrenergic C2 cell group.