Extravascular lung water and vascular permeability-surface area products were measured with a multiple indicator dilution method in 6 premature lambs with hyaline membrane disease 1–5 hours following delivery by cesarean section. The indicators used were51Cr-labelled erythrocytes,I25I-albumin,3H-water, and14C-urea. Results were compared with previously obtained data in newborn lambs without hyaline membrane disease also delivered by cesarean section. Extravascular lung water was significantly higher in lambs with hyaline membrane disease [23.2 ± 1.0 (SEM) vs. 10.7 ± 1.4 ml/kg body wt]. Vascular permeability-surface area products for14C-urea were significantly lower in lambs with hyaline membrane disease (0.30 ± 0.10 vs 0.78 ± 0.11 ml/s per kg). It is concluded that extravascular lung water is high in lambs with hyaline membrane disease. Permeability-surface area products forl4C-urea is low in lambs with hyaline membrane disease, which probably indicates a decrease in detectable surface area for exchange due to derecruitment or hypoperfusion of pulmonary exchange vessels in edematous and hypoxic areas of the lungs.