Pulmonary neuroendocrine cells (PNECs) in fetuses synthesize gastrin-releasing peptide (GRP, or mammalian bombesin) at high levels, but the role of this hormone in lung development has been obscure. The present study demonstrates that bombesin administered for 2 to 4 d toward the end of gestation in utero led to increased DNA (days 17 and 18) and saturated phosphatidylcholine (SPC) synthesis (day 18) in a dose-dependent fashion in fetal lung. These kinetics coincide with the timing of endogenous GRP gene activation in untreated fetal mouse lung, where GRP mRNA is detectable on day 16 and peaks at day 18. Electron microscopy on in vivo bombesin-treated fetal lung showed an increase in the number of cells containing lamellar bodies on both days 17 and 18, consistent with increased growth and/or maturation of type II cells. In mouse fetal lung organ cultures, the addition of bombesin led to accelerated uptake of [3H]thymidine into DNA, [3H]leucine into protein, and [3H]choline into SPC, indicating that increased growth and maturation may be direct effects. Extending these observations to another species, bombesin was found to induce growth and maturation of human fetal lung in organ culture. A monoclonal antibody to bombesin (2A11) prevented bombesin-induced increases in choline and thymidine incorporation in lung organ cultures and also blocked baseline automaturation of control lung organ cultures in serum-free medium. These data suggest that bombesin, and thus PNECs, play a role in normal lung development.