A radioimmunoassay was used to quantify substance P-like immunoreactivity in the lower brainstem of the rat. The peptide was detectable in all brainstem nuclei investigated. A very high concentration was found in the substantia nigra, and high concentrations in the interpeduncular nucleus and central gray matter. Levels in the pons, the parabrachial nuclei; in the medulla, the nucleus tractus spinalis of the Vth nerve and the nucleus of the solitary tract were higher than the brain average.