To study lung fluid balance before and after birth, we measured lung lymph flow and concentrations of protein in lymph and plasma of 22 unanesthetized fetal lambs and compared results with previous studies done on 26 newborn lambs, 1–2 wk old. Lymph flow, relative to lung mass, was less in fetuses than in newborns; lymph protein clearance was not significantly different. Less lymph flow before birth probably reflects less available surface area for fluid exchange in microcirculation of fetal lungs, compared with newborn lungs, with no difference in endothelial permeability to protein. Extravascular lung water, measured gravimetrically for 24 fetuses (10 without labor, nine in labor, five 6 h after vaginal birth), decreased by 45% (15 +/- 2 g/kg body wt) before birth and by an additional 38% (6 +/- 1 g/kg) after birth. In five lambs killed after birth, we measured lung lymph flow before and during labor and for 6 h after breathing began. Lymph flow was unaffected by labor but increased transiently after birth, accounting for 11% of the liquid removed from lungs postnatally. Liquid clearance studies performed in eight anesthetized 3-wk-old lambs confirmed the observation that lung lymphatics drain only a small fraction of liquid in potential air spaces. Most of that liquid probably leaves the lungs directly through pulmonary circulation.