The presence of an endogenous gastrin-releasing peptide (GRP)-like peptide in the hindbrain of rat was demonstrated immunohistochemically using antisera directed against the N-terminus and C-terminus of GRP. N-terminal and C-terminal-like immunoreactive material were distributed throughout the nucleus of the solitary tract (NTS), dorsal motor nucleus of the vagus (DMV) and tractus solitarius (TS), as well as in areas postrema (AP) and substantia gelatinosa separating AP from NTS. Positive immunostaining was localised to a dense network of nerve fibres which project longitudinally along the neuraxis. Immunolabelled cell bodies were observed rostral to the obex, principally in the mediolateral subnucleus of NTS. These immunopositive neurones project their axons caudally and longitudinally towards the commissural subnucleus of the NTS. Immunolabelled cell bodies also were found in AP; they projected their axons caudally and ventrally towards NTS. Positive immunostaining was blocked by pre-adsorbing antisera with either GRP (1 nmol/ml) or bombesin (3 nmol/ml), but was unaffected by substance P (30 nmol/ml) and spared by capsaicin pretreatments which deplete sensory nerves of their peptide content. The results indicate that NTS neurons containing a GRP-like peptide connect the rostral and caudal regions of the dorsal vagal complex by way of longitudinal nerve tracts descending NTS and TS. Some neurons in AP also contain a GRP-like peptide and appear to connect with the dorsal vagal complex.