A radioimmunoassay using an antiserum which recognizes the C-terminal of bombesin was used to determine the regional distribution of bombesin-like peptides in discrete regions of rat brain. The concentration of endogenous bombesin-like peptides was 30-fold greater in high (substantia gelatinosa trigemini, nucleus tractus solitarius, interpeduncular nucleus and arcuate nucleus) than low regions (caudate, hippocampus and cingulate cortex). When brain extracts were fractionated using high pressure liquid chromatography techniques two major peaks of immunoreactivity were obtained, the more hydrophobic peptide coeluted with synthetic bombesin. It is concluded that the current evidence supports the contention that bombesin may be a neuroactive peptide in the brain.