To investigate the possible mechanism of thyroid hormone action in the lung, we examined fetal and adult rabbit lung, and cell lines derived from lung, for specific nuclear binding sites for T3. Using incubation of isolated nuclei with L-[125I]T3 at 37 C, we found approximately 2400 specific binding sites/cell in fetal lung and 1120 sites/cell in adult lung, with a similar dissociation constant (approximately 500 pm) for both tissues. The L-2 and A549 cell lines, which may have originated from pulmonary type II alveolar cells, contained 2280 and 1580 nuclear sites/cell, and the dissociation constants were 280 and 200 pm, respectively. In fetal lung, the ability of various analogs to compete for L-T3 (100%) binding was: 3,5-diiodo-3'-isopropylthyronine, 81%; D-T3, 73%; L-T4, 6.7%; 3,3'-diiodothyronine, 0.19%; 3,5-dimethyl-3'-isopropyl-L-thyronine, 0.15%; and rT3, 0.08%. These findings indicate that both fetal and adult lung, and cultured lung cells, contain specific nuclear binding sites for T3, suggesting that these tissues and their type II alveolar cells may be directly influenced by thyroid hormones.