Bombesin is a tetradecapeptide with a COOH terminus ending in Gly-His-Leu-Met-NH2 and subsequently is shown to closely resemble two mammalian bombesin -related peptides, gastrin-releasing peptide (GRP) and neuromedin B (NMB). Bombesin is found to have stimulatory effects upon gastric and pancreatic secretions, release of gastrointestinal hormones, gallbladder contraction and bronchoconstriction. It is present in amphibian gastric endocrine cells, avian proventriculus endocrine cells and avian brain. In mammals it is present mainly in nerve cells and fibers. The only mammalian endocrine cell shown to date to have bombesin is the P-cell in fetal lung. Bombesin is also found in mammalian brain, with its highest concentration in the hypothalamus. Bombesin is shown to be a potent mitogen for Swiss 3T3 cells. In the presence of a low concentration (3.5%) of serum, bombesin stimulates 3T3 cell proliferation. In serum-free medium, bombesin induces DNA synthesis in the absence of any other added growth factor (IC ₅₀ =1 nM).