Recently, we found that the translocation of inhaled nanoparticles from the airspace to secondary organs is age-dependent, and substantially greater in neonates than adults (Semmler-Behnke et al., 2008). One of the reasons for this observed difference might be age-dependent differences in alveolar barrier integrity. Because the neonatal lung is undergoing morphogenetic and fluid balance changes, we hypothesize that the alveolar barrier of the developing lungs is more easily compromised and susceptible to foreign material influx than that of adults. Based on this line of thought, we predict that the postnatally developing lung is also more likely to allow the translocation of some materials from the airspace to the lymphatic lumena. To test this idea, we intratracheally instilled methyl methacrylate into immature and adult lungs and compared lymphatic filling between these two age groups. Scanning electron microscopy of the resultant corrosion casts revealed peribronchial saccular and conduit lymphatic architecture. Deep pulmonary lymphatic casts were present on the majority (58.5%) of airways in immature lungs, but lymphatic casting in adult lungs, as anticipated, was much more infrequent (21.6%). Thus, the neonatal lung appears to be more susceptible than that of adults to the passage of instilled methacrylate from the airspace to the lymphatics. We speculate that this could imply greater probability of translocation of other materials, such as nanoparticles, from the immature lung as well.