The absorption of liquid volume and radiolabeled tracer solutes from the lungs of premature lambs was investigated during the first hours of ventilation. Exteriorized lambs, 133–141 d gestation, with intact umbilical circulation had [131I]albumin and [125I] cytochrome C added to the fetal lung liquid before breathing, and, additionally, [57Co]cyanocobalamin added to the remaining alveolar liquid after 20, 60, and 120 rnin of ventilation. Alveolar liquid was rapidly absorbed from the alveolar space, averaging more than 75% absorption in the first hour whether they breathed spontaneously or were ventilated. All animals absorbed proteins and water at the same rate as water during the first 20 rnin of breathing. Older, spontaneously breathing lambs released more surfactant into the alveolar space (302 ± 82 pM) than younger ventilated animals (92 ± 32 pM). After the first 20 rnin of breathing, older animals no longer absorbed [131I]albumin and [125I]cytochrome C, and at 4 h had near normal lung wet:dry weight ratio, 5.7 ± 0.2. Younger animals continued to absorb [131I]albumin and [125I]cytochrome C as rapidly or more rapidly than liquid, and after 4 h had retained liquid in the lung wet:dry weight ratio 6.3 ± 0.2. Alveolar surfactant appears able to alter the solute permeability of ventilated premature lung and the clearance of the fetal liquid from the lungs.