Thyroid hormones influence fetal and neonatal lung growth and maturation. However, the effect of naturally occurring, genetically determined hypo- or hyperthyroidism on fetal or neonatal lung maturation has not been examined. In the hyt/hyt mouse, primary hypothyroidism, which is characterized by a high serum TSH concentration, is transmitted as an autosomal recessive trait. It occurs due to a mutational defect in the beta-subunit of the TSH receptor. We studied the lung ultrastructure of the fetal [18-d-gestation (term = approximately 19.5 d)] and neonatal (< 1-d-old) hyt/hyt mouse. In addition, disaturated phosphatidylcholine and total phospholipid contents of newborn hyt/hyt mouse lungs were determined. Male and female hyt/hyt mice with a high serum TSH concentration were made euthyroid by adding 3,5,3'-triiodothyronine to drinking water and then mated. Balb-c mice served as euthyroid controls. Fetal and neonatal hyt/hyt mice had a higher serum TSH concentration than the Balb-c controls. Fetal hyt/hyt mouse lungs showed a large amount of intracellular glycogen and fewer lamellar bodies in epithelial type II cells compared with Balb-c fetal mouse lungs. The neonatal hyt/hyt mouse also showed signs of lung immaturity such as persistent epithelial cell glycogen, few lamellar bodies, reduced disaturated phosphatidylcholine content, and absent tubular myelin. We conclude that fetal and neonatal lung maturation is delayed in the hyt/hyt mouse with primary hypothyroidism.