The clearance rate of aerosolized and deposited 99mtechnetium diethylenetriamine pentacetate provides an index of lung epithelial permeability. However, the location of the epithelium being assessed is uncertain. We determined the percentage of submicronic aerosol deposited on ciliated and nonciliated airways in healthy and damaged lungs using 99mTc-sulphur (99mTc-S) colloid. Colloidal particles can be cleared by mucociliary transport but not through the epithelium. Piglets aged 12-72 h, weighing 0.7-3.3 kg (average 1.6 kg) were anesthetized and underwent endotracheal intubation. 99mTc-S labeled submicronic aerosol (Syntevent II) was collected in anesthetic bags and either inhaled spontaneously (n = 4), or insufflated by hand (n = 5) at 30 breaths per min and a peak airway pressure of 15 cm H2O. Piglets were immediately extubated and scanned on a gamma camera. Twenty-four h later a repeat scan was performed. The residual radioactivity represents the amount of aerosol deposited on nonciliated airways. In six other piglets pulmonary damage was produced by an intravenous infusion of air microemboli for 2 to 3 h (0.14 to 0.38 ml/min). The maximal decrease in PaO2 while breathing room air ranged from 24-59 mm Hg. During the last 15 min of infusion they were intubated, insufflated by hand with the 99mTc-S aerosol, and scanned as above. The results demonstrate that approximately three-fourths of the submicronic aerosol is deposited distally to ciliated airways in both healthy and damaged newborn piglet lungs. This suggests that the clearance rate of 99mtechnetium diethylenetriamine pentacetate predominantly reflects epithelial permeability of terminal lung units when a submicronic aerosol is used for delivery to the lung.