To investigate the role of neutrophils in the pathogenesis of respiratory distress after premature birth, we assessed the relationship between circulating neutrophil concentration and neutrophil accumulation in the lung, lung lymph and pleural liquid flow, and extravascular lung water in 10 chronically catheterized preterm lambs (127 +/- 1 days gestation) that were mechanically ventilated for 8 h after birth. Circulating neutrophil concentration transiently decreased within 2 h after birth and then returned to prenatal values by 6-8 h. The decrease in circulating neutrophil concentration was related directly to the accumulation of neutrophils in the air spaces, drainage of liquid and protein from the lung 6-8 h after delivery, and postmortem extravascular lung water. In additional studies, we intravenously administered mechlorethamine to 5 fetal lambs to reduce circulating neutrophils before delivery (neutrophil concentration before birth: 9 +/- 11 cells/microl). Compared with control lambs, neutrophil-depleted lambs had significantly less drainage of liquid (7.8 +/- 5.9 vs. 2.6 +/- 1.9 ml/h, respectively) and protein (116 +/- 74 vs. 42 +/- 27 mg/h, respectively) from the lung 6-8 h after birth and significantly less extravascular lung water at postmortem (6.5 +/- 0. 8 vs. 4.8 0.6 g/g dry lung, respectively). Thus neutrophils contribute to the pathogenesis of respiratory distress after premature birth by increasing lung vascular protein permeability and promoting lung edema.