We have shown recently that apoptosis occurs during fetal and postnatal lung development. Our hypothesis that branching morphogenesis occurs through a delicate balance of cell proliferation and apoptosis predicts that substances that enhance branching of the airways would affect both cell proliferation and apoptosis in the lung. Bombesin-like peptides have a mitogenic effect on bronchial epithelium and fibroblasts, and bombesin has been shown to enhance branching morphogenesis in fetal lung. We used organ cultures of 16-day gestation fetal rat lung to study the effects of bombesin on apoptosis. Cultures were incubated in serumless medium alone or exposed to 1microM bombesin for 0-48 h. Levels of apoptosis were quantified using the TUNEL assay and expressed as percentage of apoptotic cells in paraffin sections of explants. Bombesin significantly inhibited apoptosis in fetal lung mesenchyme 48 h in culture by more than 50% (p < 0.05). The effects of bombesin on apoptosis were prevented completely if explants were exposed to the specific bombesin receptor antagonist, [D-Phe12]bombesin. To examine if the absence of serum in the media could have accounted for some of these effects, explants were cultured for 48 h in serumless medium, medium containing 10% fetal bovine serum, serumless medium with 1 microM bombesin, or medium containing both 10% fetal bovine serum and 1 microM bombesin. The addition of fetal bovine serum to the media reduced apoptosis significantly. The effect of fetal bovine serum on apoptosis was additive with bombesin. We conclude that bombesin inhibits apoptosis in developing fetal rat lung mesenchyme through its interaction with the bombesin receptor.