The pulmonary neuroendocrine (NE) cells, from 16 term infants dying at 1-4 days of age from birth asphyxia, were immuno stained for bombesin-like immunoreactivity by the immunoperoxidase method. The distribution and frequency of bombesin-immunoreactive NE cells were quantified morphometrically and correlated with the presence or absence of brainstem function and persistent fetal circulation (PFC). In infants with loss of brainstem function, the frequency of bombesin immunoreactive NE cells was significantly increased compared to infants with intact brainstem function, i.e. meconium aspiration with PFC. Infants with brainstem injury, with one exception, failed to develop PFC. Pathological changes in the tegmentum of the brainstem, i.e. containing the respiratory center, correlated in nine of 10 cases with loss of brainstem function. These data suggest an inverse relationship between brainstem function, release of bombesin-like peptide from the pulmonary NE cells and the functional state of the pulmonary vasculature. Intact brainstem function appears to be essential for both the release of bombesin-like peptide from the NE cells and for pulmonary vasoconstriction leading to PFC; absence of brainstem function is, on the other hand, associated with failure to release bombesin-like peptide and loss of PFC type reactivity in the pulmonary vasculature. However, it appears unlikely that bombesin itself is a direct mediator of pulmonary vasoconstriction.