After repair of congenital heart defects, stress responses in the pulmonary circulation of fourteen infants produced by a brief endotracheal suctioning procedure were studied before and after fentanyl (25 micrograms/kg). The total time of disconnection from the ventilator with suctioning (FIO2 1.0) was limited to no more than 15 sec to avoid alveolar hypoxia. Before fentanyl, marked increases occurred in mean pulmonary artery pressure and pulmonary vascular resistance index with suctioning, whereas only mild increases in heart rate and mean systemic arterial pressure occurred. All of these increases with suctioning were almost completely abolished by 25 micrograms/kg fentanyl. We conclude that suctioning or other broncho-carinal stimulation can produce a marked pulmonary vasoconstrictive response in infants, which is blunted by fentanyl. This response is separate from that produced by hypoxic pulmonary vasoconstriction associated with prolonged clinical suctioning procedures or with loss of airway.