Specific angiotensin II (ANG II) binding sites in the dorsal aspects of the medulla oblongata of dogs and rats were shown previously to be dependent upon intact sensory and motor components of the vagus nerve. The present studies compared the distribution of ANG II binding sites in the canine medulla with the visualization of substance P immunoreactive fibres, since this peptide is known to be present in vagal afferent fibres. Dense bands of substance P-containing fibres were found to cap the tractus solitarius (TS) in the dorsal subdivision of the nucleus tractus solitarii (nTS) at a level corresponding to the region of highest ANG II binding. In addition, moderate densities of both substance P immunoreactivity and ANG II binding sites were dually localized in the medial nTS and the ventral portion of the dorsal motor nucleus of the vagus (dmnX). Since previous studies indicated that ANG II binding sites are associated with vagal afferent fibres in the nTS and efferent motor neurons in the dmnX, their apparent association with substance P immunoreactive fibres suggests that a functional interaction may occur between these two peptides in the central integration of cardiovascular function.