We assessed sequential changes in the permeability properties of the pulmonary epithelium in spontaneously breathing, newborn term (30 days of gestation) and preterm (28 days) rabbit pups, using the rate of pulmonary clearance of 99mTc-DTPA (MW = 492) as an index of permeability. In term rabbits, clearance was faster at 1 h of age than at hourly timepoints thereafter (p < 0.05). In preterm rabbits, clearance rates measured from 1 to 5 h after birth were not quite significantly different (p = 0.0519) although the trend to slower clearance with increasing time after birth was significant. When term and preterm rabbits were compared, clearance was similar at 1 h after birth but was faster at both 2 and 3 h in the preterm rabbits (p < 0.05). Pulmonary epithelial permeability appears to be increased in the immediate postnatal period and the duration of increased permeability is longer in preterm rabbits. Because lung water content at birth is greater in the preterm rabbits, we speculate that the permeability changes may be associated with clearance of fetal lung liquid.