The pulmonary capillaries of neonatal lungs are potentially vulnerable to stress failure because of the complex changes in the pulmonary circulation that occur at birth. We studied the ultrastructure of the blood-gas barrier (BGB) in premature and 1 day old rabbit lungs and compared this with adult lungs. Normal gestation of rabbits is 30 days. After extensive pilot measurements, three premature (27 day gestation) and three newborn (1 day old) rabbit lungs were perfusion-fixed at arterial, venous and airway pressures of 25, 0 and 10 cmH₂O, respectively, and compared this with three adult lungs. The thickness of the capillary endothelium, alveolar epithelium and interstitium of the BGB was measured at right angles to the barrier at random points. A striking finding was the large number of measurements of the interstitial thickness in 1 day old lungs that were very thin (0-0.1 µm). The percentages of occurrence of very thin interstitium in premature, 1 day old and adult lungs were 35.3 ± 9.4, 71.7 ± 5.2 and 43.0 ± 2.6 (p < 0.02 for 1 day old v. premature and adult). Given the previously found relation between stress failure and interstitial thickness, this large proportion of very thin interstitial layers in the capillaries of 1 day old lungs is a reasonable explanation for their previously demonstrated vulnerability to stress failure.