We have studied the expression of genes involved in glucocorticoid synthesis in the developing lungs of male and female mouse fetuses on gestation days (GD) 15–18 (surge of surfactant, GD 17; term, GD 19). High levels of steroidogenic acute regulatory protein, cytochrome P450 cholesterol side chain cleavage, 3β-hydroxysteroid dehydrogenase type 1, 21- hydroxylase, and 11β-hydroxylase mRNAs were observed in three of the six litters studied on GD 15 and in none of the 14 litters analyzed between GD 16 and 18. Of these three litters, two showed high expression levels for these five genes in lung tissues from female fetuses only, whereas in the remaining litter, only tissues from male fetuses presented high expression of these genes. In contrast, 11β-hydroxysteroid dehydrogenase type 1 mRNA level was very low on GD 15 and presented a gradual increase between GD 15 and 18 with no sex difference. Our data indicate that, like the mature adrenal, the fetal lung expresses all genes required in glucocorticoid synthesis from cholesterol. In addition, our results demonstrate that transient expression of these genes on GD 15 in the fetal lung occurs for both male and female fetuses, 2 d before the surge of surfactant synthesis, which is stimulated by glucocorticoids.